Stand with your right foot forward, arms on guard. Throw a double jab with your left arm by quickly doing two jabs in a row. Next, throw a right cross punch, then repeat a single jab on the left. Quickly "cover" (imagine you are trying to protect your torso from your opponent's punches) by twisting your upper body (hips stay still) and bringing your right elbow to your belly button. Immediately reverse to the left, and then repeat one more time to the right (the pace of this move is very quick, try counting "1, 2, 3" as you do it to keep your tempo up). Return to start. That's one rep.
Repeat 10 times in a row as quickly as you can, and then switch your stance and do 10 reps on the other side.