Once you are ready to cook, you need to "purge" the live crawfish of mud and dirt. Put the crawfish in a large tub or baby pool and fill with cool water and 1/2 the box of salt. Once the water becomes muddy, drain and repeat the process until the water remains fairly clear. Do not allow the crawfish to just sit in the water, as this could kill them. Repeat at least 3 times if not more, although you don't have to keep using salt. (The cleaner the water, the cleaner your crawfish are on the inside. And that's a good thing.) Pick through and discard any crawfish that haven't survived the journey. I recommend using thick rubber gloves because the ones that are alive are going to be ready to pinch. Be careful not to put any dead crawfish in the cooking pot.
Cover a large table with a disposable plastic tarp and line with newspapers. Set out rolls of paper towels. Have a large trashcan with thick liner nearby for shells.
Fill water about halfway up a large steel stockpot with a strainer insert. Add seafood boil and bring to a boil for a few minutes to get the spices going. Working in batches, add some potatoes and cook until just barely tender, approximately 10 minutes. Add sausage and cook for about 5 minutes. Finally add the corn and cook about 5 more minutes. The crawfish should go in last. These should only cook for about 2 to 3 minutes, and no longer than 5. As soon as the first crawfish floats, remove the strainer from the water, allowing excess water to drain back into the pot. Dump crawfish and vegetables onto the table. Season with kosher salt and pepper and serve with hot sauce, cocktail sauce and melted butter, if desired.
Repeat in batches. Each batch of crawfish will be spicier as the water boils down. If you lose much water, add another bag of crawfish boil and more water. To eat, grab the crawfish between the head and tail, pinch and twist. The body and tail should separate. Suck the juices from the head. Peel away a section or two of the shell from the top of the crawfish. Pinch the crawfish at the base of the tail. This will loosen the crawfish from the shell. Pull out the meat and enjoy. Any leftover crawfish should be peeled immediately and frozen for another use. To clean up, pull up plastic tarp with remaining shells and dispose of immediately, double-bagging if necessary.