Start by lunging over the foam roller. It's nice to place your hands on it, for added support, similar to having a yoga block under each hand. Breath, here, deeply for 5-10 breaths.
Toe-heel your foot across the top edge of your mat, so that you are sitting on the roller. You'll notice it fits snugly into the crook behind your knee.
Walk your hands forward and start to move your torso toward the floor.
Eventually, you will fold forward completely, as in half pigeon pose. By rocking slightly forward and back along the roller, you'll feel added support and emphasis in your hip, hamstring, and outer thigh. The roller will also help adjust your shin toward parallel with the top edge of your yoga mat (the fullest expression of the pose). Be sure to flex your front foot to protect your knee, and if you feel any twinges in the knee, back off right away. Hold for 10 breaths or more on each side.