Sit on the ground with your hands on the floor supporting you. Place the foam roller underneath your calf (you can start anywhere but it can be good to start a third of the way up your calf from your foot (the top of the Achilles tendon, and then move upwards). Relax your leg as much as you can so the muscle relaxes over the roller.
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. Stay for about 15 seconds then bring your body closer to the roller so the roller starts an inch higher up your calf and repeat. 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.