1.0 TL;DR
2.0 Name
3.0 Skill Tags
3.1 Devotion Skills
4.0 Skill Point Costs
5.0 Mana Costs
5.1 Action Costs
2.0 Name
3.0 Skill Tags
3.1 Devotion Skills
4.0 Skill Point Costs
5.0 Mana Costs
5.1 Action Costs
1.0 TL;DR
I typically make skills piece by piece in the order of Effect->Action Cost->Mana Cost->Prerequisites/Skill Point Cost (which are actually the same thing)->Skill Tags->Name. A skill should cost 1 Skill Point unless it's real good, or 0 if it's real un-good. Pride of Giants is meant to be fun more than it is meant to be balanced, err in the players' favor. If a skill does a thing that another skill or thing does, make sure they don't stack in a way which makes your tummy hurt, and if they're the same type of skill (Martial and Martial for example) consider making one a Prereque for the other. Do as I say, not as I do: don't make Mana Costs too high, 2 is good if the skill gets used a lot and it's a good skill, 4 is pretty high, 6 is a whole mess of Mana, be careful. Pride of Giants is meant to be fun more than it is meant to be balanced, err in the players' favor. The key words are "useful" and "interesting" a skill should be at least one, and a good skill should be both if it can help it. GL, HF.
2.0 Name
Name: A Skill's name is irrelevant. In PoG skills' Names have one of a few purposes, often to make the Effect intuitive without having to read it (for example the Necromancy skill Stitch Flesh), to relate the skill to lore (for example the Chronomancy skill Chronologian's Exchange), or to sound good (like the Martial Skill Indiscriminate Elegance).
3.0 Skill Tags and Devotion Skills
Skill Tags: A skill's Skill Tags are usually irrelevant, except for some prerequisites. For example, skills may list "1 Neuromancy Skill" or "1 Divination Skill" as Prerequisites. If it is not irrelevant, you can check each type of skill's pages for description of what they are, if need be. For example, at the very top of Divination Skill's page, it says "Divination Skills have to do with magically gaining information and foreseeing outcomes and possibilities." Thus, if a skill would have to do with gaining information, it would be a Divination Skill, even if was not also, for example, a Magical Skill.
3.1 Devotion Skills
When creating things, the details of what you create should usually either be useful or interesting. It is not useful for a skill to be Devotion Skill if the character would not become "over powered"/too powerful compared to the other players' characters by taking another Devotion Skill. It is not interesting for a skill to be a Devotion Skill if the skill disrupts the player's experience by being a Devotion Skill.
The Transmutation Skill Shape Shift is no longer a Devotion Skill because a player of mine wanted to have multiple scale colors as a dragon, but also wanted to be able to shape shift to fit through doors. As the Host, it was neither useful nor interesting to me for one of my players to not be able to fit through doors in a dungeon delving game, and it would not have been interesting for the player to be made weaker (and fulfill their character to a lesser extent) simply so that they could play the game with everyone else, even if it would have technically been useful for them to fit through doors, and to be able to deceive others before dropping shape shift and dramatically revealing their true form.
In another game, the same player wanted another character to have the Divination Skill True Sight for flavor reasons, and to fulfill a couple of role play interactions. I deemed that it was useful to me for that player to be able to role play in that way not only for narrative reasons, but more importantly because my player wanted to have fun in that way, and that fun does nothing to disrupt others' fun. However, the character was heavily combat focused, and so would be put behind other combat focused characters by taking that Devotion Skill instead of others, punishing the player for their fun. It was useful for me that True Sight not be a Devotion Skill in that context, and it was not useful for the player for that skill to be a Devotion Skill, so I simply gave it to the player as a regular skill, and also reduced the Skill Point Cost to 0 because it hurt nothing to do so, and made the player happy.
If it is not useful or interesting for a skill to be a Devotion Skill, it should not be a Devotion Skill. If you decide for a skill to still be a Devotion Skill while it isn't useful or interesting for it to be, the reason for this decision may be perceived to be spite or negligence.
The Transmutation Skill Shape Shift is no longer a Devotion Skill because a player of mine wanted to have multiple scale colors as a dragon, but also wanted to be able to shape shift to fit through doors. As the Host, it was neither useful nor interesting to me for one of my players to not be able to fit through doors in a dungeon delving game, and it would not have been interesting for the player to be made weaker (and fulfill their character to a lesser extent) simply so that they could play the game with everyone else, even if it would have technically been useful for them to fit through doors, and to be able to deceive others before dropping shape shift and dramatically revealing their true form.
In another game, the same player wanted another character to have the Divination Skill True Sight for flavor reasons, and to fulfill a couple of role play interactions. I deemed that it was useful to me for that player to be able to role play in that way not only for narrative reasons, but more importantly because my player wanted to have fun in that way, and that fun does nothing to disrupt others' fun. However, the character was heavily combat focused, and so would be put behind other combat focused characters by taking that Devotion Skill instead of others, punishing the player for their fun. It was useful for me that True Sight not be a Devotion Skill in that context, and it was not useful for the player for that skill to be a Devotion Skill, so I simply gave it to the player as a regular skill, and also reduced the Skill Point Cost to 0 because it hurt nothing to do so, and made the player happy.
If it is not useful or interesting for a skill to be a Devotion Skill, it should not be a Devotion Skill. If you decide for a skill to still be a Devotion Skill while it isn't useful or interesting for it to be, the reason for this decision may be perceived to be spite or negligence.
4.0 Skill Point Costs
All of a skill's costs should be determined primarily by how useful a skill is, where usefulness is a measurement of how frequently a skill can be used, and how effective the skill is (within its specific context: for example Subserve Savagery (discussed below) is considered extremely useful, even though it is of no use at most tea parties, because Subserve Savagery's context is combat). Skill Point Cost should be primarily determined by specifically how frequently a skill is useful.
If a skill has a Skill Point Cost of 0, then the skill likely isn't particularly useful, even if it is tremendously powerful. Maybe it allows you to make it rain on command, but your character in particular would never be in a situation where this could make you the savior of some desert village, or where you could capitalize with lightning damage, it just makes you the person who can make things wet. Maybe a 0 Skill Point skill allows you to tell if someone's a god on sight, but it's only really useful the one time you meet a god, and it's more of a conversation starter than a huge boon anyway. A 0 Skill Point skill may be useful if it's also extremely niche. For example, a skill which lets you permanently change the color of anything you touch. Cool, stylish, but when will that be like... "Good"? In order for that to be an "important" skill, a player will have had to have maneuvered and planned something out maybe sessions ahead of time, at which point they've earned it, and it shouldn't cost them anything, maybe 1 Mana per activation, maybe not.
A skill should have a Skill Point Cost of 1 unless otherwise stated. Four of these such skills can be taken per level.
A skill with a Skill Point Cost of 2 must be frequently useful and offer an identifiable advantage to the character taking it. Two of these such skills can be taken per level, which will make for a somewhat uneventful level up. Such skills must compensate by being so useful that their worth half a level's investment.
A skill should only have a Skill Point Cost of 3 if it is very powerful and useful, but not powerful enough to warrant a full level of investment. This is a fine line to tread, and I avoid it whenever possible. When in doubt, make it cost 2 skill points.
A skill which costs 4 skill points, one full level, must be nearly character defining. A skill which could be a Devotion Skill, given the right circumstances. A skill which offers a huge advantage, or huge potential advantage. This should also be kept in mind when making any skill which has a cumulative skill point cost of 4 or 5 when you add their Prerequisites' costs together; they must useful enough to warrant a full level's investment either together or separately.
A skill which costs more than 4 skill points is likely several skills standing on eachothers' shoulders in a trench coat, such as the Elemental Skill Elementalist or the Offensive Magical Skill Black Mage, or some great boon which circumvents a mechanic of the game, such as Create Phylactery, which removes death as a mechanic for the player who takes it, or Unlife for a Life which removes death as a mechanic for any character other than the one who takes it.
If a skill has a Skill Point Cost of 0, then the skill likely isn't particularly useful, even if it is tremendously powerful. Maybe it allows you to make it rain on command, but your character in particular would never be in a situation where this could make you the savior of some desert village, or where you could capitalize with lightning damage, it just makes you the person who can make things wet. Maybe a 0 Skill Point skill allows you to tell if someone's a god on sight, but it's only really useful the one time you meet a god, and it's more of a conversation starter than a huge boon anyway. A 0 Skill Point skill may be useful if it's also extremely niche. For example, a skill which lets you permanently change the color of anything you touch. Cool, stylish, but when will that be like... "Good"? In order for that to be an "important" skill, a player will have had to have maneuvered and planned something out maybe sessions ahead of time, at which point they've earned it, and it shouldn't cost them anything, maybe 1 Mana per activation, maybe not.
A skill should have a Skill Point Cost of 1 unless otherwise stated. Four of these such skills can be taken per level.
A skill with a Skill Point Cost of 2 must be frequently useful and offer an identifiable advantage to the character taking it. Two of these such skills can be taken per level, which will make for a somewhat uneventful level up. Such skills must compensate by being so useful that their worth half a level's investment.
A skill should only have a Skill Point Cost of 3 if it is very powerful and useful, but not powerful enough to warrant a full level of investment. This is a fine line to tread, and I avoid it whenever possible. When in doubt, make it cost 2 skill points.
A skill which costs 4 skill points, one full level, must be nearly character defining. A skill which could be a Devotion Skill, given the right circumstances. A skill which offers a huge advantage, or huge potential advantage. This should also be kept in mind when making any skill which has a cumulative skill point cost of 4 or 5 when you add their Prerequisites' costs together; they must useful enough to warrant a full level's investment either together or separately.
A skill which costs more than 4 skill points is likely several skills standing on eachothers' shoulders in a trench coat, such as the Elemental Skill Elementalist or the Offensive Magical Skill Black Mage, or some great boon which circumvents a mechanic of the game, such as Create Phylactery, which removes death as a mechanic for the player who takes it, or Unlife for a Life which removes death as a mechanic for any character other than the one who takes it.
5.0 Mana Cost and Action Costs
All of a skill's costs should be determined primarily by how useful a skill is, where usefulness is a measurement of how frequently a skill can be used, and how effective the skill is (within its specific context: for example Subserve Savagery (discussed below) is considered extremely useful, even though it is of no use at most tea parties, because Subserve Savagery's context is combat). Mana Cost should be primarily determined by specifically how powerful or effective a skill is within its context, for example being higher on a skill which deals a high amount of damage (such as the Martial Skill Heart Breaker which deals 100 True Damage in exchange for 100 Mana) or a skill which is highly useful in certain situations, (such as Instant Shift which allows you to quickly activate Shape Shift, which may also instantly end any chase as the people chasing you are still looking for what you looked like before you turned to corner, or simply to shape shift and then still have your Primary Action for combat. This makes it potentially very useful, and specifically useful within all of the contexts in which it would be used, giving it a very high total Mana Cost of 10). Lastly, a Mana Cost should be somewhat based on Action Cost.
Action Cost and Mana Cost, when added together, are the total cost of the activation of a skill. Each action type has its own value and purpose.
To summarize Actions: Primary is the main thing you do in a turn because it is strong enough to be worth that, Defensive is for preventative defense and for powerful modifications to Primary Actions, Reaction is for reactive defenses and for actions which are taken off of your turn, Move is for moving yourself, or others, or other things, or for modifications to magic, making spell casters less mobile (such as Arcane Bolt which can be activated again as a Move Action), Bonus is for additional actions and actions which modify other actions, Extra is for instant things like drawing a weapon, or harmlessly tossing an item, and Free is highly conditional, both for quicker than instant actions, and for skills which a player should be able to activate, either to maximize a skill's usefulness (such as Double Tab, discussed below) or because it is a Reaction which can't cost a Reaction (such as Cyclical Violence, briefly mentioned below).
Action Cost and Mana Cost, when added together, are the total cost of the activation of a skill. Each action type has its own value and purpose.
To summarize Actions: Primary is the main thing you do in a turn because it is strong enough to be worth that, Defensive is for preventative defense and for powerful modifications to Primary Actions, Reaction is for reactive defenses and for actions which are taken off of your turn, Move is for moving yourself, or others, or other things, or for modifications to magic, making spell casters less mobile (such as Arcane Bolt which can be activated again as a Move Action), Bonus is for additional actions and actions which modify other actions, Extra is for instant things like drawing a weapon, or harmlessly tossing an item, and Free is highly conditional, both for quicker than instant actions, and for skills which a player should be able to activate, either to maximize a skill's usefulness (such as Double Tab, discussed below) or because it is a Reaction which can't cost a Reaction (such as Cyclical Violence, briefly mentioned below).
Mana should probably be called 'energy' or, like, 'calories', but Mana is way more standard, so I went with that. The more effort a skill takes to activate, the higher the Mana Cost. The more powerful the skill, the more effort the character has to put into making it happen.
If a skill can be activated effortlessly, and there are no consequences if a player activates the skill an infinite number of times, then it has a Mana Cost of N/A.
If some effort is involved, but there are no consequences if a player activates the skill an infinite number of times, then it has a Mana Cost of 0.
If a character is meant to activate a skill several times, such as across multiple turns and/or over the course of concentration, and it is not a particularly powerful skill, it should have a Mana Cost of 1 or 2 (or 1* or 2*, as the case may be for concentration skills).
12 Mana goes from a lot of Mana early in the game, to a still noticeable amount of Mana later in the game, so I frame a lot of Mana Costs in my head around multiples of 6. A skill which costs 4 Mana can be activated 1 additional time with 12 Mana over a skill which costs 6 Mana to activate. If a skill is useful, but not highly powerful on its own, it should probably have a Mana Cost of 3 or 4. If a skill is powerful (which is to say good at solving problems, or a skill which deals unusually high damage) it should probably cost 5 or 6 Mana, such as the Martial Devotion Skill Subserve Savagery which grants +1 Attack at the cost of a Bonus Action, or the Martial Skill Double Tap which grants +1 Attack as a Free Action when you reduce a target to 0 HP. If a skill's usefulness is primarily limited by the creativity of its use, it is likely to have a Mana Cost of 7. A skill which is very powerful is likely to have a Mana Cost in the teens such as 12 or 14, though may have a condition for reducing its cost.
If a skill can be activated effortlessly, and there are no consequences if a player activates the skill an infinite number of times, then it has a Mana Cost of N/A.
If some effort is involved, but there are no consequences if a player activates the skill an infinite number of times, then it has a Mana Cost of 0.
If a character is meant to activate a skill several times, such as across multiple turns and/or over the course of concentration, and it is not a particularly powerful skill, it should have a Mana Cost of 1 or 2 (or 1* or 2*, as the case may be for concentration skills).
12 Mana goes from a lot of Mana early in the game, to a still noticeable amount of Mana later in the game, so I frame a lot of Mana Costs in my head around multiples of 6. A skill which costs 4 Mana can be activated 1 additional time with 12 Mana over a skill which costs 6 Mana to activate. If a skill is useful, but not highly powerful on its own, it should probably have a Mana Cost of 3 or 4. If a skill is powerful (which is to say good at solving problems, or a skill which deals unusually high damage) it should probably cost 5 or 6 Mana, such as the Martial Devotion Skill Subserve Savagery which grants +1 Attack at the cost of a Bonus Action, or the Martial Skill Double Tap which grants +1 Attack as a Free Action when you reduce a target to 0 HP. If a skill's usefulness is primarily limited by the creativity of its use, it is likely to have a Mana Cost of 7. A skill which is very powerful is likely to have a Mana Cost in the teens such as 12 or 14, though may have a condition for reducing its cost.
6.0 Breakdown
Lets take a look at two skills, Double Tap and Subserve Savagery. Both are Martial Skills with the same end result: you gain +1 Attack. The difference arrives with which skill is more useful when considering usefulness to be how often a skill can be used to its fullest extent. Subserve Savagery can be used to its fullest extent by activating it with a Bonus Action. To use Subserve Savagery beneath its highest possible potential requires the player to make a mistake, making it a very easy skill to use; Subserve Savagery is always useful in situations where its effect (gain +1 Attack) is capable of being useful.
Double Tap, however, could be nearly useless, depending on how combats work in a given game, and at its highest potential Double Tap will be somewhat useful, allowing you to potentially damage a specifically already weakened/defeated enemy. Double Tap is the most useful in battles where there are several enemies, and battles where those enemies aren't necessarily disabled by being brought beneath 0 HP, such as against Monsters and Humans, assuming your Host doesn't have enemies just drop out of battle at 0 HP. Double Tap is rarely useful and when it is, it still may not be.
Subserve Savagery
1a. Subserve Savagery has a highly powerful effect (Gain +1 Attack.)
1b. And so has a high Mana Cost (5)
2a. Subserve Savagery is always useful in its context (if Attacks are useful)
2b. And so has a Skill Point Cost of 3.
3a. Subserve Savagery stacks with a similar effect already in the game (Berserker which has the same effect, but as a more expensive/Defensive Action instead)
3b. And so has Berserker as a Prerequisite, bringing its total Skill Point Cost to 5 (Berserker's 2 + Subserve Savagery's 3).
4a. Subserve Savagery is always useful, and also very powerful, and also stacks with another skill which is frequently useful, and also very powerful.
4b. and so is a Devotion Skill.
5a. Subserve Savagery is a Devotion Skill, and it cannot be taken in the same level as its singular Prerequisite with 4 Skill Points.
5b. and so gains -1 Skill Point Cost, bringing its Skill Point Cost down to 2, and its total cost with its Prerequisites down to 4.
6a. Subserve Savagery has a very cheap Action Cost (Bonus Action), making it easy to use, and even more useful.
6b. and so gains +1 Mana Cost, making its Mana Cost very high.
Berserker
1a. Berserker has a highly powerful effect (Gain +1 Attack.)
1b. And so has a high Mana Cost (5)
2a. Berserker is frequently useful in its context (if Attacks are useful), so long as defense isn't more important.
2b. And so has a Skill Point Cost of 2.
3a. Berserker has an expensive Action Cost (Defensive Action) which may be better used on a Reaction, particularly in Parry heavy builds.
3b. and so gains -1 Mana Cost, making its Mana Cost average, and penalizing its use less.
4a. Berserker will be a staple skill for some characters, being used every single turn.
4b. I make the skill Inevitable Frenzy which makes Berserker free to activate, but you always have to activate Berserker if you have Inevitable Frenzy.
5a. Berserker is not very useful for characters who Parry.
5b. I make the skill Cyclical Violence which effectively grants another Parry as a Free Action with no Mana Cost, but instead has the cost of an enemy gaining a free attack against you.
Double Tap
1a. Double Tap has a highly powerful effect (Gain +1 Attack.)
1b. and so has a high Mana Cost (5)
2a. Double Tap is infrequently or rarely useful
2b. And so has an Action Cost of Free so that it can be used as frequently as possible, and has a Skill Point Cost of 1 with no Prerequisites. It would not be unreasonable for certain characters to gain this Skill with a Skill Point Cost of 0, or even to integrate it as an Innate Action, but I don't want to have every 1 cost skill cluttering up PoG's Core Rules.
Double Tap, however, could be nearly useless, depending on how combats work in a given game, and at its highest potential Double Tap will be somewhat useful, allowing you to potentially damage a specifically already weakened/defeated enemy. Double Tap is the most useful in battles where there are several enemies, and battles where those enemies aren't necessarily disabled by being brought beneath 0 HP, such as against Monsters and Humans, assuming your Host doesn't have enemies just drop out of battle at 0 HP. Double Tap is rarely useful and when it is, it still may not be.
Subserve Savagery
1a. Subserve Savagery has a highly powerful effect (Gain +1 Attack.)
1b. And so has a high Mana Cost (5)
2a. Subserve Savagery is always useful in its context (if Attacks are useful)
2b. And so has a Skill Point Cost of 3.
3a. Subserve Savagery stacks with a similar effect already in the game (Berserker which has the same effect, but as a more expensive/Defensive Action instead)
3b. And so has Berserker as a Prerequisite, bringing its total Skill Point Cost to 5 (Berserker's 2 + Subserve Savagery's 3).
4a. Subserve Savagery is always useful, and also very powerful, and also stacks with another skill which is frequently useful, and also very powerful.
4b. and so is a Devotion Skill.
5a. Subserve Savagery is a Devotion Skill, and it cannot be taken in the same level as its singular Prerequisite with 4 Skill Points.
5b. and so gains -1 Skill Point Cost, bringing its Skill Point Cost down to 2, and its total cost with its Prerequisites down to 4.
6a. Subserve Savagery has a very cheap Action Cost (Bonus Action), making it easy to use, and even more useful.
6b. and so gains +1 Mana Cost, making its Mana Cost very high.
Berserker
1a. Berserker has a highly powerful effect (Gain +1 Attack.)
1b. And so has a high Mana Cost (5)
2a. Berserker is frequently useful in its context (if Attacks are useful), so long as defense isn't more important.
2b. And so has a Skill Point Cost of 2.
3a. Berserker has an expensive Action Cost (Defensive Action) which may be better used on a Reaction, particularly in Parry heavy builds.
3b. and so gains -1 Mana Cost, making its Mana Cost average, and penalizing its use less.
4a. Berserker will be a staple skill for some characters, being used every single turn.
4b. I make the skill Inevitable Frenzy which makes Berserker free to activate, but you always have to activate Berserker if you have Inevitable Frenzy.
5a. Berserker is not very useful for characters who Parry.
5b. I make the skill Cyclical Violence which effectively grants another Parry as a Free Action with no Mana Cost, but instead has the cost of an enemy gaining a free attack against you.
Double Tap
1a. Double Tap has a highly powerful effect (Gain +1 Attack.)
1b. and so has a high Mana Cost (5)
2a. Double Tap is infrequently or rarely useful
2b. And so has an Action Cost of Free so that it can be used as frequently as possible, and has a Skill Point Cost of 1 with no Prerequisites. It would not be unreasonable for certain characters to gain this Skill with a Skill Point Cost of 0, or even to integrate it as an Innate Action, but I don't want to have every 1 cost skill cluttering up PoG's Core Rules.