Sit on an abdominal board and place your feet under the pads or sit on the floor with your feet under an immovable object. Bend your knees comfortably and keep them bent throughout the entire set.
Lie back onto the board or floor with your arms across your chest or hands behind your head.
Curl your upper body up until it is at angle slightly less than 90 degrees to the floor.
Put all your attention into the abdominal area tensing it as hard as possible for a count of 3. 
Keeping your abs as tense as possible, slowly return to the starting position.
Keep maximum tension on the abs and repeat the motion.
After failure, rest 90 seconds and do a set of conventional sit up to failure.