How do you date your girlfriend's best friend?
The first step is obviously to dump your girlfriend. Which makes the next step really, really hard, particularly if they are close friends. Tread carefully, my friend! It isn't a pretty path.
-Take it very slow! If you go to fast your gf's best friend might think that you only dated her best friend to get her, which may cause her to think your a player, or just not to like you. It also depends how close the two are. And, obviously, it depends if her best friend likes you or not!