We probably boil nearly a thousand pounds of a combination of crabs, crawfish and shrimp a year. Mostly for family get-togethers and neighborhood things. We catch our own crabs and shrimp. All of our old crawfish spots have become "salty". And, there are no longer any crawfish there. Gotta buy them now.
We use a powdered product from a company "Zatarain's" for our boil. I'd bet that Stickman's link is pretty close to the ingredients in the purchased stuff. Rather than buy all of those things, I just buy the one. Down here, every little corner grocery store carries it. In addition to this I add more salt, cayenne powder, halved onions, halved heads of garlic and for extra stuff to eat, potatoes, corn-on-the-cob, cauliflower etc. Phil uses lemon and oranges in his boil. I use very little. My wife really likes extra lemon so, I quarter-up some for her to squeeze onto her food. I never cook any meats in with the shellfish since I don't like my crawfish or crabs all greasy. I do smoked sausage in a separate boil.
As for amounts. I make the boil-water twice as salty, twice as pepper-hot, twice as garlicky etc..as I want my shellfish to be. For crabs I allow them to actually BOIL for about 6-7 minutes, crawfish 3-4 minutes, shrimp as soon as the water starts to boil, kill it. Let all of them soak for 30 min to1 hour. The fatter they are, the longer it takes for the seasoning to penetrate into the shellfish.
Another method is to have some 2-liter bottles of ice ready ahead of time. Keep testing your shellfish for "doneness". As soon as the texture/peel-ability is where you want them, kill the burner, and pitch in your bottled ice. The soak times can be cut a little shorter with this method since the cooling effect makes them absorb the seasoning quicker. Do not dump plain ole ice into your boil since it will dilute your seasonings too much. I really do like this method but never have room in the freezer or, I never remember to save the bottles and to make the ice ahead of time.
We always have on the table saltine crackers, ketchup, hot-sauce and horseradish for those who enjoy a cocktail sauce with their seafood. And, we have butter for the potatoes and corn.