We usually wash them, purge them, wash again, purge again, then wash a final time.
All the while we've had the pot going coming up to a good boil. We usually use Louisiana Fish Fry Boil Seasoning. Let that get going, dissolved and such. Usually throw the corn, red potatoes, and sausage in before the crawfish and let those cook separate.
Once the sides are done, we dump another half package of boil seasoning into the pot (for people who like less spicy) and fill the pot up with crawfish. Let those go awhile, once done put in the cooler and sprinkle them with dry seasoning. Shake the cooler around and let those rest.
Start all over, another half bag into the boil as well as a little hot sauce (crystals, Louisiana, Tabasco, whichever you prefer). Add the crawfish, let them go for a while as everybody who doesn't like the spicy stuff starts eating. Once the spicy ones are done, put them in a separate cooler with some dry seasoning sprinkled on top. Shake it up and let them rest.
By this point you've probably had a 4-5 beers and been snacking on sausage, corn, and potatoes. Drink another beer while the spicy ones rest and then enjoy.