Setup and Key Actions
Lie on your back and hug your right knee in toward your chest—closer than 90 degrees so that there's no gripping in the hip flexors. Keep your left leg extended, heel on the floor, toes pointing up toward the ceiling, or plant your left foot flat on the floor. Externally rotate your right thigh slightly so that your right kneecap points toward your right armpit/shoulder. (This will prevent the right knee from collapsing in, which it tends to do in this pose, and allows for more stretch in the outer hamstrings, which often get overlooked.) Bind your right big toe with your right index finger, middle finger, and thumb, or catch hold of the pinky-toe side of your foot with your right hand. Slowly start to straighten your leg keeping the thigh close to the torso. Only straighten to the point where you feel the stretch primarily in the middle part of your hamstring (not up toward your sitting bone or down toward your knee). Draw the pinky-toe side of your right foot down toward the floor, and extend out through the ball of your right big toe. If your left leg is extended, draw the pinky-toe side of your left foot back toward your calf, and extend out through the ball of your left big toe. Press your sitting bones down toward the floor to maintain a small space between your lower back and the floor.
If you're unable to comfortably catch hold of the pinky-toe side of your right foot, wrap a strap around the arch of your foot and hold on to the strap. If your left (bottom) leg really wants to turn out, try practicing at a wall, with the sole of your left foot pressing into the wall, which can help it to stay put!