Fill a washtub or ice chest with water and 2 cups of the salt, stir, and place the crawfish in the mix. As they swim around, the salt will cause them to purge themselves of impurities and cleanse their outer shells. (Let them purge for 30 to 40 minutes; they need to stay alive until you're ready to cook them.)
While that's happening, pour the 12 gallons of water into a 20-gallon pot. Add the remaining salt, the seasoning, cayenne pepper, black peppercorns, bay leaves, and lemons. Boil for 15 minutes. In a basket insert, place half the garlic, half the onions, and half the potatoes. Place the basket in the water and cook for 4 to 5 minutes. Add half the corn and return the water to a boil.
Drain the crawfish from their purging water, add half of them to the basket, and bring to a rolling boil. Turn off the heat and let soak for 10 minutes. Pull out the basket, drain, and dump the basket's contents onto a newspaper-lined table. Repeat with the remaining ingredients.
Serve crawfish, corn, onions, and garlic to each person.
Thoroughly combine all ingredients in a blender, food processor, or mixing bowl, and pour the mixture into a large glass or plastic jar. Seal it so that it's airtight. It will keep indefinitely.
You certainly can boil a small amount of crawfish in a smaller vessel, do it indoors, and serve it to a smaller gathering, but in New Orleans, this is for an outdoor bash.
Recipe courtesy of Ti Martin and Jamie Shannon, From Commander's Kitchen, Broadway Books 2000.