[header] How to do a stability ball knee tuck [title] Put your exercise ball in an open area. [step] You should have plenty of free space to move around, and all sharp/heavy objects should be removed so as to minimize the potential for accidents. [title] Lie face-down on the ball and walk yourself forward on your arms. 
[step] As your torso comes off the ball your legs should roll onto it, so that you end up with your shins pressed into it and the rest of your body supported by your arms. Make sure to keep your head aligned with your shoulders and hips.