you find a flat surace that you can reach one of your legs past (A door frame, or for example when I'm at the gym a pillar) and you place the leg you want to stretch straight up the flat surface so that you both your heel and which ever arse cheek the leg is attached to are flat up against the surface. You have to make sure that your knee joint is fully extended other wise it won't work. You do the same with the other leg. you extend it fully and kep it flat on the floor. It helps if you have some one to keep your foot upright because untill your loose enough its quiet hard to keep the ankle twisted.

keep your back flat on the floor. Basiclly all you're doing is making a right angle with your legs.

Its a powerfull stretch, just wondering if you think that its putting to much stress on certain areas etc.