A 40 pound sack of crawfish is one of the few things you don't cook in cast iron. There are no cast iron crawfish pots; They would be too heavy and expensive to be practical.
Since boiled crawfish is so readily available in Louisiana during crawfish season, I never needed to boil my own. But I decided that I wanted to learn. Boiling crawfish is not hard, but it is very involved.
First, some common sense safety. Place your burner outdoors, on level ground, on pavement (not grass). Sweep the area around the burner, and create at least 6 feet of clearance space. Have a fire extinguisher handy. Keep the propane hose away from the flame. Never leave the burner unattended while it's lit.
WEAR GLOVES when handling live crawfish, because they will bite! Have a spare pair of gloves in case the first one gets wet. Wear old clothes, because you will have crawfish goo all over you by the time you're done, including in your hair!
Have an ice chest with cold beverages; It gets pretty hot out there, and you'll use some of the ice in the cooking process.
You need a propane burner that's at least 100,000 BTU. Most outdoor fryers, turkey fryers, etc. are 50,000 BTU or less, and are not designed for crawfish boiling.
You need a huge pot, with a basket. If you want to boil a full sack all at once, you need an 80 quart stock pot. Aluminum is cheaper, but stainless steel is easier to clean, lasts longer, and aluminum doesn't leach into the food. You could also use a 40 quart pot and do 2 batches.
You need a basket lifter, and you need some kind of a rack to set the basket on to drain. Most people use 2 pieces of wood. You need a "crawfish paddle" to stir with. Metal tongs are also handy.
You need an outdoor table, covered with newspapers to dump the crawfish on when you're done, and a large, lined garbage can by the table.
You need a hose and outdoor faucet, in order to wash off the crawfish, and fill up the boiling pot.
You need something to put the crawfish sack in, to wash off and soak/purge. This will need to be designated for crawfish use only, since it will forever smell like crawfish.
Ideally, you would use a large ice chest with a drain, but I used a 30 gallon plastic bin. You don't need a drain plug; Just keep the crawfish contained in the sack until they're cleaned, although a drain plug would be more convenient.
And then you need the "extras". Salt, onions peeled and quartered, garlic cloves, celery, lemons, and seasoning such as Zatarain's crab boil or "pro boil".
You can put other stuff in the pot to boil too, like corn, potatoes, and sausage. Some even add a whole chicken! Just keep in mind that the more extras you have, the less room you'll have for crawfish.
Now go buy your crawfish. Bring CASH because many fresh seafood merchants do not take debit cards. Full sacks were $45; prices will vary. Please check your sack to make sure it's tightly closed, because live crawfish will crawl out-- they actually chewed a hole in my sack!
First, put the sack on the pavement and run the hose over it. Then put the sack in the bin, and continue hosing down the sack. Dump the dirty water out, hose some more. Repeat until the water runs clear. Then fill the bin with enough water to cover the sack. Pour salt in the water-- 1 box per sack. Let soak for 3 to 5 minutes, then dump out the salt water and hose the sack down again.
You don't want to leave crawfish immersed in water for too long. They will drown. After you've hosed off all the salt water, you can empty the sack into the bin and watch them scurry around. If you find any dead ones, throw them out.
Now, the actual times for soaking and boiling is VERY much up for debate. I ended up boiling for 7, soaking for 20.
Fill your pot 2/3 full with water. Add your seasoning. Put the onions and garlic in the basket and lower it in. Squeeze the juice from the lemons into the water and throw the peels in.
Light your burner, place the pot on the lit burner, then turn the heat up slowly. Taste the water; if it's not spicy enough, add more seasoning. Cover and bring the water to a rolling boil.
If you're adding potatoes and corn, add those and let them cook for several minutes before adding the crawfish. But I didn't do that this time.
Now you're ready to add the crawfish. Just grab them by the handful and throw them in-- WEARING GLOVES. Use the paddle to push them farther down, and keep adding crawfish until the pot is full.
Let the crawfish boil for 7 minutes, covered. Then take the lid off, turn off the burner, hose down the sides of the pot, add ice*, and let the crawfish soak 15 to 20 minutes.
*It's important to cool the pot after boiling because that allows the crawfish to soak up the spices.
Lift the basket out of the pot and set it on the rack to drain. After it's cooled enough to handle, dump the basket on the table and eat.
Unpeeled crawfish doesn't keep in the refrigerator for more than a couple of days. You have to peel and freeze whatever you don't eat that day. I'll be making crawfish pie soon!