LOVE LOVE LOVE this.
-Ly on your back.
-Now bend your knees accordingly:
Knees should be pointing up, feet off the ground. Basically, your thighs are pointing vertical, and below the knee is horizontal.
-Put your hands behind/on your ears. (its important not to use your hands to pull your head up)
-'crunch' your abs, while touching your right elbow to your left knee. This should be a crossing motion.
You should be pulling your knee slightly to meet your elbow, as the other knee briefly and slightly straightens.
-The switch. Touch your left elbow to your right knee.
This targets your side abs as well as lower (around the belly button and under)
Hope this helps & good luck.
PS. your tummy with get sore fast, which means it's building muscle. Do about 20 in a row, then ly flat to take a breath. Then do 30. Breath. 20. Breath....and so on.
Doing this to music, or on the floor in front of a good movie or tv show, helps distract you and makes you able to 'workout' better & longer.