Okay...this is quite a tough exercise. I personally don't like this exercise but hey...if it is good for the back...just do it.
Lie down on your stomach.
Put both hands at your back, holding it together if possible.
Then raise both legs and hands together.
It should be that only your abdomen that still touches the floor while the other parts of the body are raised up.
Put your leg and hands down (and head) and repeat.