Before each semester begins the department will send email to all the grad students about the position needed for teaching assistant. If you want to apply for one position, just write an email to the professor who provide that position. The professor will schedule a meeting with you for an interview. Usually interview question includes past experience and basic problems about the course.

Talking to a professor and usually taking their class. There’s not that much else but I need to get this to a 30 word minimum now so here it goes.