In this demonstration you have your opponent in the side mount and will attempt to get the mounted position with the knee drive method.
Grab your opponents belt with your right hand and slide your right knee onto their stomach. As you do this your opponent tries to push your knee off their stomach with their left hand.
Let go of your opponents belt and grab their left wrist. At this point your opponent may try to pull their arm away, simply pin their hand to the ground with your right hand.
Take your right knee off your opponents stomach and take your left hand under your opponents left bicep and grab your right wrist as shown.
Switch your knee positions so that your right leg is layed straight with your left knee against your opponents body.
Step over your opponents head with your left foot. This will prevent them from sitting up to escape the arm-lock.
To apply the arm-lock slide your opponents hand up along their body and raise your right elbow.