I dunno if there is any specific way you have to do it. My mage mastered all Collages except for Conveyance. I just went to the guy and he asked me do you wanna be a master and I said yes.
Yeah, how? As I understood it, you could only master one college at a time. Meaning if you master a second (or third, or fourth, etc) school, you'd lose the previous bonuses.
You can only master one College of Magick. If you need more than that, it's time to move on to another game. But stick around here for the fun!