basically, get on your knees, and go as "far as you can" but don't overextend yourself so that you can't get back up, and then come back up trying to use your ab muscles.
do three sets of 20, and you "should" start feeling some sort of a burn.
As long as you're careful, and you're in relatively decent physical shape, I think you'll be ok.
Very similar to this video:
YouTube - Great AB Exercise For Six Pack Abs