Yeah, I'll get more details to you this evening, but its basically a two-part process. A caster has spell slots (a 3rd level mage has some 2nd level slots and some 1st level slots, for example). Making an item requires the use of those spell-slots for a period of time, depending on the item's power... more powerful items need higher level spell slots. If the mage can sacrifice the slots required for the required time, he can make the item. That's one part, the "mechanics" of it. It might sound more complicated than it really is. I'll try to post something more specific later on, when I get to my books.
The second part is component based. This is the "RP" portion of item creatin, in my games. Magic items require rare and powerful components not always available at the bazaars or magic shops. Players must acquire these components somehow, either hunting down the pieces themselves, paying someone else to do it, or tracking down a merchant who deals in such esoteric items. This also allows GMs to limit the kind of magic items he wants in his games. If you don't want to alllow rings of invisibiliy, make the components difficult or impossible to get.
Thats the gist of it, anyway.