Picture A shows how. The most important thing when stretching your calf is not to turn out your back foot. Take a small lunge with the toes in line with the heels so that the feet are in parallel position, otherwise there will be no stretch in the back calf. Keep your back upright and your hips forward (in line with your shoulders).
To stretch the lower part of your calf and down into your Achilles tendon, slide the back foot forward, about h half way towards the front foot and bend both knees, again with your back upright and hips forward. See Picture B. Change legs.
Stretch slowly (about 30 seconds each stretch), relax and breathe.