Place the foam roller underneath your calf (you can start anywhere but it can be good to start at the top of your calf and then move downwards. Relax your leg as much as you can so the muscle relaxes over the roller.
With the other foot, very slowly push the roller forward to it rolls down the calf. You can then pull your toes up toward you and then point them away from you (bend and flex your foot) to make the calf muscle slide backwards and forwards over the roller. Aim to take around 2-3 minutes to get right down the calf. You can continue these steps until you have released your whole calf.

Progression: Have the non-released leg resting on top of the other one to provide more downwards pressure (as shown in the picture). Lift your bottom off the ground by pushing down through your hands. Try to not let shoulders lift up near your ears.