Make sure you have a large enough pot to boil or steam the crabs in. I recommend a 10-20qt pot with a built in steamer insert (as pictured below). Fill up enough water to cover the crab if boiling the entire crab or just below the steamer to properly steam the crab.
Place the crab into the pot holding the back two hind legs pulled up or using tongs. Close the lid and cook crab (boil or steam) for 8-12 minutes (depending on how many crab and size of pot). Pull the crab out with a pair of tongs and place under cold running water to chill the crab down to prepare for the next step.
Find the crab's abdominal flap on the underbelly of the crab. Pull it down and away from the crab to separate the flap from the body. You can also pull off all the the legs and claws as well leaving just the body.
To remove the body of the crab from it's carapace you must hold the top of the shell while pulling the body down away from the shell. (Example shown above)
The final step is to discard the shell and pull the lungs off of the body. These are filled with the most toxins and I cannot stress enough how sick it can make you if you eat these lungs!
It's easy to clean if you run the body under water while removing the lungs. There is also a mushy center which holds a lot of the crabs innards (or "butter") which I personally will also wash out under running water while running my thumb inbetween the center of the crab.
This will leave you with a large fist sized body which can be split in half and LIGHTLY broken apart. If you break the body exoskeleton too much the little bits can break off into the meat and present a unpleasant eating experience. Also, eating the "butter" in excess or for those with strong tendencies to allergic reactions need to be careful. Keep up with current health advisories before consumption always!