Extend one leg in front of you with the foot flexed. Bend your other knee and lean back slightly. Your pelvis should be tilted forward. Keep your upper body upright as you hold the stretch for 10-20 seconds, then switch sides.
You should feel the stretch up the back of your extended leg (all the way up your calf and thigh).
Your supporting leg may get tired, as you have to balance your weight on it.