Working with fresh crabs can be awkward. They're highly perishable once they're killed, and require a degree of cleaning and preparation. If they're still alive they'll defend themselves vigorously, at some hazard to your fingers. Often it's simpler just to buy frozen crab instead, cleaned and precooked at the processing plant while they're at their peak of freshness. Because they're fully cooked already, they only need gentle reheating before they're eaten.
Run 2 to 3 inches of water into the bottom of a large pot. Bring it to a boil, and fit it with a steamer insert. If you don't have a suitably large steamer insert, use a wire trivet or cooling rack instead, and balance it on egg-sized balls of aluminum foil to keep your crabs above the water level.
Arrange the whole crabs loosely in your steamer, leaving plenty of room for the steam to circulate. If you have a large quantity to prepare, do them in batches rather than overloading the steamer.
Cover the pot. When steam begins to billow freely again from under the lid, begin timing. Steam the frozen crabs for 4 to 7 minutes, depending on their size, until a thick leg is hot all the way through when you snap it off and crack it open.
Remove your steamer from the heat and remove the lid, taking care to avoid the steam. Use tongs to lift the crabs out onto a serving tray. Serve them as is, or remove the upper shell and feathery gills first.