Don't understand skills

[Atty]

For example the Peer Fortress has a skill "Gatekeeper's Proof: You provoke the enemy, meaning they'll focus on you and not hit your teammates as much."

How do I use this skill? I Can't find any skill menu in battle. Also each skill has a cost. What is this cost? It is not explained anywhere I can see.

[Landale]

Most skills are passive. That skill, like anything that mentions being Donum, are accessed through the Donum option in battle.

The cost on each skill is how many points, before discounts, it takes to equip.

[Atty]

How do you know how much points you have in total to equip?

[Landale]

The quickest way is to go to the character's status page, then the skill tab. This is where you can freely equip and unequip learned skills. At the top of that tab is also your skill point total as well as how many are used.
You start with 40 points, gain 1 for each soul clarity over 1 up to 99, 1 for every 10 clarity over 99 up to the cap of 999, and there are two pacts that give 10 and 30 points respectively allowing for a total of 268 skill points by the end of the game. Unless you grind in excess though, you're unlikely to get more than ~130-150 skill points by the time you finish the game and you honestly don't need more than that.

[Atty]

Okay thanks for the explanation!