You dont use the forms , you use the abilities you get from them, like how you can slide from wisdom form... you get to glide after leveling up final form
Like the valor form.... i think it levels up based off of hits right? so you just stroll around a lot in valor form in a world with lots of heartless. smack the bejeebers out of them and hit a save point to recover gauge.
for wisdom form i went to disney castle and I went to TimelessRiver place.