You can perform the exercise with the bar on your shoulders or on your chest. Lets show how its done traditionally on the back. With your arms grabbing the bar at a wide distance, lower the bar onto your shoulder blades. Put your toes on either a big weight plate, or a low step.
Slowly push up onto the very tip of your toes and at the top pay total concentration to your calves. Then slowly lower yourself down past when your feet are horizontal until you feel your calves starting to really contract. Pause at the bottom and then start the next rep.