3 sets, hold each side for 30 seconds.

Sit with one leg straight out in front of you. Keep back straight and chest upright while you lean forward slightly from hips until stretch is felt in the back of your leg.

There is an option to use a strap around the ball of your foot. Pull your foot forward into a flex position with the strap, and hold tension with while leaning as described above.

You could also do attain the same stretch by instead standing. Place the leg to be stretched on a stool or stair then lean forward toward your leg as described above.