But wait, there's more! You need a catalyst in order to attach MATERIA. These can only be obtained by the Disciple of the Land classes. And as you guessed it, depending on what type of item it is depends on what type of catalyst you need. And the different Disciple of the Land classes garner different catalysts suited for the varied item types.

Or you can simply ask someone to affix Materia for you. But be warned, the first MATERIA is free, the next on the same item may result in complete destruction!

Good luck!