Wash them and make sure there are no dead ones. Then, throw them into boiling seasoned water and let them boil for 3-8 minutes. The duration of the boil depends on the pot size, how much crawfish went into the pot, and how hard the crawfish are (early in the season the crawfish are softer and should be boiled less, later in the season the crawfish are harder and should be boiled longer).
A good rule of thumb: Boil until crawfish rise from the bottom of the pot. Turn heat off and let them soak for 30 minutes. Try one and add seasoning to taste. BON APPETIT!!!