Choose an appropriate weight. Sit on the machine seat and lean against the backrest. The balls of your feet should be firmly on the platform and your heels extending off. Take a shoulder-wide stance. Grasp handles with both hands.
Straighten the legs by extending the knees, i.e. contracting calf muscles (m. gastrocnemius). As you breathe out, press downward through the balls of your feet as far as possible. As you release, lower your heels just slightly below the platform and breathe in.
Avoid rotation of your toes and heels to the sides.
Keep your heels and toes at one level.
As you start, choose an appropriate resistance. It is recommended to stay in top position (maximum calf muscle contraction) for 1 second.
Just like the leg press, this exercise is making use of machine designed to avoid the pressure on your back by the loaded barbell.
In fact, such machines are even more beneficial for calf workout: while in the leg workout you are somewhat losing on limiting the movement (as compared to squats), this is not the case with calves.
There are several types of leg machines but the exercise is basically the same using any of them.