Ask her to the movies. See the type of movie in between what you would both normally see. Don't see something she's dying to see because if she's too interested in the movie their will be no sexual tension build up. Wait about half the movie to put your arm around her UNLESS she gives you cues that she wants to be touched, or if she touches you a lot first. In that case touch her right away, put your arm around her etc.
Kiss her goodnight.