You progress from easy to hard as long as there is no pain. You can go from sitting heel raises with longer holds to adding weight to progression to standing with 2 legs to 1 leg. That can further be progressed by standing with the balls of your feet resting on a step, dropping your heels drop below the level of the step then use your calves to push up on your tip toes.
There are many machines in gyms that can facilitate this by increasing weight. You can ride a stationary bike pushing with the balls of your feet.
Remember that the strongest leg will always dominate if you are doing strengthening using 2 legs.
Unfortunately the girth of calf muscles takes a while to increase but you should notice the ability to do more with time, indicating strength return