You are in the mounted position and your opponent straightens their arms, giving you the perfect opportunity for an arm-bar.
This demonstration shows how to apply the arm-bar on your opponents right arm. Place your right hand on your opponents chest in between their arms.
Take your left arm round the outside of your opponents right arm and place it on your right hand. This is isolating the arm you are about to perform the arm-bar on.
Put all of your weight on your hands and hop to your feet as shown.
Now still with all your weight on your opponents chest, swing your left foot round the top of your opponents head to finish in the position shown.
Now simply sit down next to your opponent. You should aim to be as close as possible to their right shoulder. As you sit down you will noice that you have your opponents arm in a perfect position to apply the arm-bar.
Grab your opponents right arm and lay back. To apply the lock simply raise your hips off the ground, just an inch of movement should cause pain to your opponent. Be sure that your opponents thumb is pointing up so their arm is perfectly locked. Also squeeze your knees together slightly for a solid hold.