Using a squat-rack, place the pins at a level where you can comfortably un-rack the bar. Place the bar on your rear shoulders and comfortably grip it with your hands. Un-rack the weight and take a few steps back. With your feet planted firmly on the ground, drive your body up by standing as high on your toes as possible. Squeeze your calves at the top of the movement and then lower your heels back to the ground. Continue the motion until you reach muscular failure and re-rack the bar.
A spotter should definitely be used for this lift in case you lose your balance. This exercise should be approached with caution as it will be difficult to safely use a considerable amount of weight.