Calf machine
Most modern gyms will have a calf machine, if not that use either a Smith Machine or Barbell across your shoulders.
Stand straight with the balls of your feet on the platform, with the shoulder weight cushions comfortably placed on your shoulders, - avoid being bent up, or straining to reach the supports.
Lift and lower slowly for 2 - 3 sets of 10 - 12 reps working for a full range.