Zatarain's is a great product. I'll use it if I have access to it. However, you can also cheat by using pickling spice in a tea ball or sachet.
I do a shrimp boil by getting the boil going with water, a little white wine, a couple cut lemons, and the seasoning mix in a sachet. Let this come to a boil, reduce the heat to simmer, and cook for a while, about 30 minutes.
Now, when you go to cook your shrimp, your cooking time will depend on how big they are, and whether they're frozen or thawed, and what kind of ratio of "court-boullion" (liquid) to shrimp, as the more liquid to shrimp you have, the faster they cook. This cooking time will come with experience after cooking a couple batches.
Add your shrimp, stir, and cover. I like to leave it on a low burner, although some folks leave it on a high burner, or take it off the heat completely, depending on how fast the shrimp cook and how much time you have available to watch the pot. DO NOT LET IT BOIL AGAIN! If it boils, the shrimp will be overcooked and tough. Just as the water comes back to a gentle simmer is about right, although sometimes it's done at a poach. There's only one way to check, and that's to fish one out, and butterfly it almost completely. If the shrimp is opaque throughout, it's done. Immediately drain and submerge in ice water to stop the cooking.
I've been cooking shrimp for cocktail for years. You'd be surprised at how fast I can cook 100 lbs of shrimp.
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