How do I do a ball lying triceps extension with alternating arms?

Lie on a stability ball with the ball placed under your stomach.  Extend your legs and point your feet down.  Hold a dumbbell in each hand with your elbows bent, being careful not to shrug your shoulders.  draw in your navel and extend one elbow until it is parallel to the side of your body.  Hold the top position and then reverse the movement as you return to the starting position.  Repeat with the opposite arm.