Lie on your front with your feet over the edge of the bed.
Place a weight around the ankle of your affected leg.
Place a folded towel under the thigh of your affected leg to increase the stretch.
Keeping your hips firmly on the bed, try to straighten your affected knee.
You will find you can increase the stretch by pushing your foot down further with your good leg.
Do not let your hips come off the bed as you straighten the knee.