I know when to transform characters, but how, exactly, do you transform..

Well you need a certain item. Hero crest changes fighters,myrmidons,and mercenarys. Guiding ring changes magic and or staff users. Elisian whip changes winged units. Knights crest changes knights and cavaliers. Orions bolt changes archers and nomads. Ocean seals change pirates. Fell contract changes thiefs. Master seals can change any unit exept for your lords. And heaven seals change your lords exept your main character, he will automaticly change after chapter 27.

Long story short just get the item you need (push the R button and it'll tell you what it works on) In the battlefield push the "A" button to select and hit use. You then get an option (assuming you're here for sacred stones of corse) or you can go into "Manage items" and use it there.