You being with their right knee on your right side. Your elbows are in tight and your knees are brought up and heels planted. In this way you’re active, ready to move, not flat and dead.
You turn all the way away to your left and bring your right elbow in close, further than their knee, That way, once you turn back towards them, the elbow helps to peel the knee off your body.
You bring your lower leg (your left) out far, 90 degrees, and plant it. Like a reverse shrimp, you pressure that heel and your right shoulder. You first pull your hips out in line with your leg, then turn back into them, peeling off the knee on the way, and ending up in an all fours position, your head inside their calf, both arms wrapping low by their ankle, shoulder straight into their calf against the joint. You pull the ankle towards you, shoulder into the shin, and twist head from the inside (where it is) toward the outside. These motions together will lock their leg straight and get the sweep.