The same way you would if it was one of your buddies and you wanted to talk about something you guys shared an interest in.
In a stadium seating situation, there is nothing wrong with turning around and saying, "Hello, I'm FT, what's your name?" with your arm over the back of the chair. Be sure to shake her hand and make eye contact. If you can get conversation going, invite her to come sit next to you, so you " can hear her better and not get a kink in your neck". Sometimes that's not feasible, and it is a good sign if you can get her to lean down and speak in your ear. Incidental touch during this time is extremely arousing...especially when her lips graze your ear.
If the venue is not conducive to conversation, and she can't move,arrange to meet her outside to continue it. " I want to talk to you more about this assignment. Let's talk right after lecture, and hammer out the details. Number?" Then text her during the talk (phone on silent) with your name.