Grasp the live crab by the back legs and drop it into the water headfirst. For a more humane method, as you grasp the crab by the legs, stroke the top of its head until it falls asleep and then slowly drop it into the boiling water.
Once the water has started to boil again, reduce the heat and simmer for the required time. The crab's shell should turn a bright orange when done.
When cutting open the crabs, remember to remove the limbs and discard them
Crabs are also delicious cold.