Lots easier if they are wearing mma gloves.
From classic armbar position with their head on your left side. Consider if their near hand (right) is palm up or palm down.
If it is palm up;
-hook their right arm with your left arm. Blade of your forearm just below their wrist joint. Of course be tight to control the arm. You want their elbow stuffed in your crotch to help.
-Using your right hand, reach all the way over and grab their right hand on the thumb-side. Grab the lump of flesh below the thumb.
-Bring your right elbow down to your hip rotating their palm downward and inward toward their forearm.
-With your left hand grab your own right wrist like a Kimura grip.
-Bring your elbows to your hips squeezing everything down and into your body to lock the wrist.
Im supposed to being studying for an exam tomorrow, I'll do the palm down later.