Select the item in the normal item menu, and then select one of your characters. They will then permenantly learn that spell (or until you bin it). It starts at level 1, which is usually utter junk, and levels up the more you use it. Note that at certain levels it will only start significantly gaining "experience" if you start fighting stronger monsters, but I still recommend levelling up your spells regularly. FYI you can only learn 15 spells maximum, so try to give yourself a good spread. As a general rule though make sure everyone has Cure, Life and Exit. Max Cure as much as possible, but try to keep both Life and Exit as low as possible.