Here's a general overview of the entire sequence. Start by diving underneath your partner for X-guard. (Note: having them stand up is the easiest way to start working on the back takes.) Naturally, as your opponent stands here, you're immediately going to think that you should stand up in base ("technical stand up") and do a very simple sweep. However, your opponent can lean forward to make this sweep extremely difficult, and that's the time to sneak out the back.

Control their belt with your left hand (assume that your left leg is hugging their leg with the initial X-guard entry). Next, pass your left hand behind your partner's knee (like a "karate chop"), and use this space to slide all the way out the back door. You should finish with both butterfly hooks behind your partner's knees, able to pull them forward into your lap to finish the back take.