The first step in an old fashioned crab boil is to gather your crabs. You can buy bushels of live crabs in many Eastern markets, and on the West Coast, you can sometimes find live Dungeness crabs. These here are Pacific red crabs, rock crabs and Dungeness, collected off a jetty in Northern California.
Figure on at least a half-dozen blue crabs per person, eight red crabs or calico crabs, 10 rock crabs (most of the meat is in the claws), or 2-3 Dungeness crabs. These are general guidelines, so adjust to your eaters' preference.
While the briny sweetness of the crabs themselves will flavor your meal, I always add a spice mix to the boil. While you could add anything you'd like, I prefer one of two seasoning mixes: Old Bay crab boil, or Zatarain's crab boil. Old Bay is the classic for Maryland or Virginia crab, while Zatarain's is a Cajun seasoning.
And if you are on the ocean, there is nothing wrong with boiling your crabs in seawater -- so long as it's clean.
At any rate, once your water is boiling away, drop your crabs in one at a time. Don't overcrowd the pot. Boil in batches if you need to.
The water will drop below a boil when you put the crabs in, so cover and let it come back to a full boil. When it does, cook for 10-15 minutes. When you see crabs floating on the surface, give it another 2-3 minutes.
Once the crabs are ready, carefully extract them with long tongs and set them aside to cool. You can start picking out the meat in a few minutes, or you can wait until they cool completely.
You can also store and freeze whole crabs once they are cooked this way. Cooked crabs will stay in good condition for up to a week in the fridge.