How do I perform a ball lying external rotation?

Lie on your stomach on top of a stability ball with your legs extended and toes on the ground.  draw in your navel and hold a dumbbell in each hand with your arms extended in front of the ball.  Contract your glutes and lift your chest off the ball.  Be careful not to arch your back or allow your neck to jut forward.  Row your arms by pinching your shoulder blades together.  Rotate your shoulders down and back so your hands are at ear level.  Hold this position and then reverse rotate your shoulders so that your hands are pointing down toward the floor.  Repeat the exercise, rotating your arms in and out.