In this video I show a very simple Butterfly Guard Pass. It's so simple in fact that most people dont expect it and thus it has a fairly high success rate (the first couple of times you use it).
The best part about this move is that it doesn't require a lot of practice. Do 20 or 30 reps before trying to pull it off in live rolling but make sure to practice it outside the view of your sparring partners. Otherwise they'll expect it and it probably wont work.
With that being said there are two really important things.
1. Make sure that you have a heavy downward pressure on the knees with the palms of your hands. The grip MUST be on the outside of your opponents arms so that they are out of the way. When you pop up to your feet the pressure on their legs should keep them in place for the 1 second it takes for you to transition.
2. The MOST important thing is to lead with your pelvis. I call this the Lemur Monkey. It looks very weird to let your pelvis clear the legs before any other part of your body. This is not something that you typically do in day to day life but it does happen from time to time while grappling. If you have practiced this during the Motorcycle Pass or some of the other techniques that I have posted then it should be very easy.
Reference the video to see what I am talking about.