HOW: Sit at the edge of a table or some sort of elevated surface. Ideally, you don't want your feet to touch the ground. Place your unaffected leg under your affected leg, fully supporting it. Slowly let your affected knee bend by lowering your feet. At some point, you will feel a gentle stretch. Hold for the desired amount of time, and then use your unaffected side to help straighten your knee out once again. Repeat.
FEEL: If you are doing this after surgery, it is normal for this to feel a bit uncomfortable. There should never be any sharp pain, but aching is fine.
COMPENSATION: Relax! Use your other leg to support your affected side.