Start: Attach the straps that come with the machine to the high cable, and sit on the seat with your back to the weight stack. Hold the straps at your shoulders. If it's a single strap or a rope handle, use a neutral grip; if it's two stirrup handles, use a supinated grip.
Finish: Most beginners just move their shoulders down. Be more effective by first pulling your belly button in toward your spine. Then try crunching. You'll have to use less weight ad a shorter range of motion. And you'll probably feel as if someone were probing your abdominal muscles with kitchen knives. Don't worry about the sharpness of the contraction-that's what those muscles are supposed to feel like when they're really working.