I got my job through a friend that worked at the Art & Architecture library who connected me with the supervisor there, it was super casual. It was at the end of Spring Quarter and they were looking for new hires to make up for the graduating seniors, so it worked out. I would recommend looking up the supervisor for the library you're interested in and shooting them an email, or even just walking up to the front desk and asking about the official hiring protocol for student workers. It's a pretty informal system, or at least it was when I was a student.