Lie down on your stomach on the floor. You can place a pillow underneath your stomach. Bend your knees to 90 degrees, so that the soles of your feet are pointing to the ceiling.
Try to keep your upper body as relaxed as possible. Clench your bottom so that your knees rise from the floor. Now move your knees away from each other and then slowly back together again. Make sure your knees touch each other in the end.
Repeat the movement a few times. Warning! The movement will tone your bottom as well. You should try it to believe it.