The image above shows the starting position for this exercise. If you have trouble balancing, then use your free arm to hold onto something stable. To start, bend you knee and hold you ankle behind you as shown above.
To stretch the quadriceps, simply bring your knee down towards the floor as you bring the heel closer to the buttocks. Stop when you feel a comfortable stretch. Don’t overdo it. More isn’t necessarily better. Hold the stretch for 25 seconds and then gently relax the leg. Do the same on the other side. Do a total of 3 stretches on each side, once or twice a day.