See A-Z of rehabilitation exercises
In case you cannot see the video, here is a still sequence -
The left leg is the one being stretched. Stand comfortably next to a wall, and support yourself with one hand on the wall.
Keep your weight on the right leg while you stretch your left leg behind it.
Bend the right leg (in front) to apply stretch to the left leg (at the back). Hold the stretch for a few seconds.
Bring the legs back to the starting position.
To stretch the other leg you will need to turn round and face the other way.