User blog comment:KodachiZero/Chain Chronicle V3 on the Wiki + Q&A/@comment-122.55.57.66-20161125094300/@comment-359947-20161201012729

It took me a while to understand how to equip them because - after you get their affiliation to 10+ and their levels to 40+ you need to put them in the same party. After a few fights they'll teach/learn the skill. I kept looking for a place to equip the skill. If you need to change someone's equipped skill, go to the tab in their arcana info marked Denju and click the button. It will remove the skill. After that, repeat the battle thing with the new mentor+trainee.

Also important to note, it doesn't override the other character's default skill. It adds a green button to the left of the skill bar that you can toggle skills with. Very cool to have the option!