Get Cursed Seals for your characters in the Cursed Seal Wheel (in Supreme Treasury).
You can also get materials to upgrade them and Runes to embed in your Cursed Seals.
The Function can be found in the Exercise tab. There you will find the tab for each function: Partners, Akatsuki, Kaguya, etc., as well as each Cursed Seal according to the functions.
Once you have obtained a Cursed Seal, equip it to a character of the same function.
Each Cursed Seal has a specific Skill.