Hold a button / pad direction down for a couple seconds and a menu will pop up allowing you to assign skills, potions, etc to them. The actions assigned to the A and B buttons can sometimes get triggered when finishing or exiting out of conversations, etc, so it would be better not to assign skills with a long cooldown or high mana requirements, just in case.
FWIW, playing a warrior I assigned X melee attack, A range attack, Y Rush Attack, B whirlwind, left Dragon Stone, right Dragon Morph stone (both after getting the battle tower), up Heal (level 1 comes in handy) and down summon creature (after getting it).
For potions, I generally just paused (or had the low hit point auto-pause kick in) and hit a shoulder button to bring up a similar menu as with assigning button actions. That is also handy for skills you only use occasionally.
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