Summarize:

You don't want to use a pot that is too shallow or a saucepan. That could easily make a mess. A 6-quart pot is a good idea. A sturdy pot or a steamer that is tall and high will work well. You need a steaming rack or separator on top of the pot so the crabs don't touch the hot liquid. You can purchase a steam rack or make one out of tin foil. The basic goal is to allow the steam to rise into the crabs while preventing the crab from touching the boiling liquid below it. A crab steamer is a two-part pot. The bottom pot holds the steaming liquid, while the inner pot is perforated with holes and holds the crabs. If you do not have a crab steamer, you can improvise and use a large pot and a circular rack to place in the bottom of the pot for the crabs to sit on during the steaming process. Water works to steam live crabs and frozen crab legs, but if you want to steam crabs like a local, you could try the Maryland approach, and use beer and vinegar. Add about cup salt to the water. All you need is two cans of cheap beer and an equal amount of apple cider or distilled white vinegar. Some people also add bay leaves into the mixture. If using water, pour 2 cups of water into the pot and add 1 tablespoon (14.8ml) of salt. Some recipes call for adding spices to the beer or water (such as salt, garlic, chiles, limes, and cilantro). Place about two inches of liquid in the bottom of the pot, enough to boil and produce steam. You don't want the water line to rise above the steam rack. Boil the liquid. Other recipes call for one cup of water and one cup of vinegar. You should use tongs to do this, especially if they are still alive. Put three to four crabs belly down on the steam rack. Cover them with a spice blend. Then, add another layer of crabs and repeat the seasoning process. Then cover the lid of the pot. You can find many recipes for different crab steaming spice blends online. A spice blend can vary by taste but often includes a mix of celery salt, dry mustard, cumin, black pepper, rock salt, and nutmeg. You could also purchase a spice blend for crab meat at many grocery stores.
Use a tall high-walled pot. Add liquid to the pot. Place the crabs in the pot.