Place about 1/2-1 inch of water on the bottom of a extremely large steamer pot. Add about 1 cup of vinegar to the pot. Place the steamer bottom on top of the water. The water should not go over the steamer rack.
Start layering the crabs with the seasonings. If you are cooking blue crabs, layer the crabs shell up. If you are cooking Dungeness crabs, layer the crabs shell down. Then add salt (liberally), cayenne pepper (to taste -- more will be spicier, but put at least a little even if you don't want spicy), black pepper (liberally), yellow dry mustard (moderately), and Old Bay Seasoning (liberally). Continue layering crabs and seasonings until you are done or fill the pot.
Turn the heat on the pot, cover and cook for about 45 minutes. Let cool for about 10-20 minutes or just Serve!!!