To do crunches, the movements are more practical than doing sit ups because crunches can be done without having to support the legs.
Positions that can be done to do crunch by lying down then lift the knees to the feet and the floor will form a triangle. Then push the upper body forward until it forms a 30 degree slope from the floor. You can do it repeatedly until your body feels it is not strong enough to do it. You can do this routine every day to get maximum results.