Here's a better look at the finishing worm-guard position. The lapel is fed through, and the right hand is gripping palm down, thus securing the right leg completely in the snare of this insane guard.

Start the sweep by building up your base to your left elbow. Then pull your left leg (the bottom leg) back, and extend your right leg forward.

This is not unlike a scissor-sweep motion, something most jiu-jitsu practitioners are familiar with (if you're not familiar with a scissor sweep, you probably shouldn't worry too much about worm guard at this stage in your game).

The one key difference here is that you can lift your hip off of the ground due to the built-up base from earlier, so you have a great deal of leverage. If you drop your hips down now, you should end up on top in the mount.