Secure a leg ride. From the top, hook your right leg inside your opponent's right thigh, or left leg inside of his left thigh, and use your toes to cup his ankle.
Reach across and grab the arm opposite the side that your leg ride is on, then pull it back and up so that you can slip your head under it, at or just above the elbow. While leg riding, you will be at a diagonal to your opponent, so attacking the arm across from your leg ride is natural, since it is sitting right in front of you. The difficult part is overpowering your opponent's arm so that you can get your head under it.
Use your head to lift your opponent's arm and turn him. As you turn him, keep grabbing the wrist of the arm you are attacking, and snake your free arm--the one on the same side as your leg ride--underneath his arm and around the back of his head, so that your palm is resting across the back of his neck.
Rock backward, so that your opponent goes to his back, and has his head laying over the arm that you placed behind his head. Now the arm that you attacked originally will be pinned under your body.
Release the wrist of the arm that is pinned and bring that arm over the top of your opponent and lock it with the arm you have underneath his head. With your arms locked, straighten them out as much as possible to tighten the guillotine. Keep squeezing until the referee stops you. Remember to keep your leg ride hooked in throughout the move, as it prevents him from rolling out.