You have your opponent in the mount and they have their left arm bent.
Grab their left wrist with your left hand. Cup over the top of their wrist using your thumb, this makes it hard for them to move their arm away. Also tuck your left elbow into their neck.
Take your right hand under your opponents left bicep and grab your left wrist. Use the same cupping technique as before.
Place your head on your right hand and apply pressure to prevent your opponent lifting their hand up. Now slide their wrist down the side of their body whilst raising your right elbow.