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Great question.
Standard crunch performed on a stability ball with control and without weight.
The exercise must be done on a ball suitable to your height or you may be prone to compressing your spine if fully draped on a ball too small in size. Please see below for a size chart:
Your Height Ball Size (max. height/diameter)
less than 5' 0" 45 cm (18 in.) 5' 0" to 5' 5" 55 cm (22 in.) 5' 6"- 6' 1" 65 cm (26 in.)
6' 2" - 6' 8" 75 cm (30 in.)
6' 9" and up 85 cm (34 in.) above size info taken from:
www.simplefitnesssolutions.com
Support your head with your finger tips, have both feet placed flat on the floor with a majority of your weight on your heels.
Descend back, feeling your abdominal wall stretch, pause at a point that's comfortable, and ascend back to the top, refraining from reaching a point that allows your abdominal muscles to disengage. Do the reps slowly enough and with enough intensity to reach failure by the 12 rep. Hit 3 sets total with 45 seconds of rest in between.
Hope this helped! Good luck!
Links: www.simplefitnesssolutions.com