Whats the best way to ask her out?

Back when I was pledging for my fraternity a few months ago, I had to give people rides to and from parties. One girl seemed kinda of nice but a little drunk and she said she'd remember my name and friend me on facebook. She added me probably 2-3 weeks later, but I had no idea it was even her until she im'd me weeks after that and said it was. Anyway we've talked a few times on facebook, and have gotten kind of friendly. We both go to the same university, but I haven't actually seen her since I gave her a ride home. I feel like she likes me so I want to ask her maybe if she wants to meet up to eat sometime, but I have no idea how to ask her. Any help?