It's pretty easy!! Basically when you send in your confirmation and deposit to SMC, they send you something called a "New Student Guide" which is all the information you'll need and any forms you need to fill out. One of which is the roommate survey!! It's really thorough, asking you questions like, "Do you take showers at night or in the morning? Do you like background music or quiet when you sleep? Are you cluttered, messy, or clean?" etc. Res Life does a great job of matching people up with compatible roommates- my freshman year roommate and I are still very close and it was a good match!!
However, let's say you form a connection at an Open house or on the Class of 2016 facebook page and you really want them to be your roommate, you can request them on the roommate form as well- good luck :)
Feel free to ask me anything about SMC :) www.formspring.me/alexsmc14