Ah yes, the good old girl staring you down from across the room. When this happens you need to make sure to be on a talking basis with her and have a few long conversatoins with her, but you said you flirt with her so it looks like you've got that down pat.

You can either find a time when its just you and her, and just during a convo with her go in and kiss her. Or, just ask her to see a movie with you as friends, see where that takes you.

Remember, 100% of boys are rejected by girls that they never even ask. In other words, not trying is a sure fire way to get rejected.