Begin by sitting on the ball, now slowly walk your feet forward until just your lower back is on the ball, the top of your bum should still be in contact with the ball though
Place your fingers on your temples and slowly raise your shoulders up off the ball and towards your knees
Keeping your hands where they are, return your shoulders back to the ball
Repeat steps 2 and 3 until you can no longer maintain correct technique
Tips
The position of the ball is very important for this exercise, if its too far forward, it will be very hard, not far enough the exercise is too easy and wont get any gains from it