Lunge out with one of your legs and use the opposite hand to block the heel and keep the leg bent at 90 degrees.  Try and keep you chest up nad your belly and back tight as you try and touch your butt to the ground.  When you are comfortable in this position start to walk your hands around so that your belly button faces 1st the outside of your knee and then walk it in so that it faces your foot. Execute the stretch on both left and right.