HOW: Lie on your back close to a wall. Place your affected leg on the wall. Make sure your heel can slide on the wall, so wear a sock or place a pillow cloth or something else slippery around your foot so that it can slide. Slowly let gravity slide your leg down. As your leg slides down, your knee will bend more, which is the goal of this exercise. Bend as far as you can, hold that stretch for a few seconds, then return back to the starting position. Repeat.
FEEL: You should feel a slightly uncomfortable stretch at the end of your range. This is common and the goal of this exercise is to feel that little bit of discomfort so don't be alarmed.
COMPENSATION: Let gravity bring your foot down as it slides against the wall, don't try to move it yourself. Go slow!