You have your opponent in your guard and they have their right hand on the ground.
Uncross your feet (if using the closed guard) and put both feet on the ground. Grab your opponents right wrist with your left hand.
Come up on your left elbow and take your right hand round the back of your opponents right arm. Grab your left wrist with your right hand.
Now roll onto your right side and take your opponents arm over their back. Put your left leg on their back to prevent them from rolling forwards out of the lock.
You could also hook your opponents leg to prevent them from rolling forwards.
A close-up of your hand positions, to apply the lock simply push your left hand to the right of your opponents head.
A perfect position to be in, here your opponent has layed down making it nearly impossible for them to come back up to their knees to roll out of the arm-lock.