Start with your back and arms flat on the mat, knees bent and feet planted on the floor.
Move it up to the highest level of difficulty by raising one leg up until it's vertical. Contract the abs and butt for 1-2 seconds, just enough to lift your hips a couple of inches further; relax without changing your stance. It's easier if you imagine you're reaching to the ceiling with your foot. Repeat 5-10 times each side.