Fill your pot, with the basket in it, to the fill line with clean water. Add salt until the water tastes like the ocean, then add the Zatarain's packets (and liquid if using), garlic, onion, and lemons. Squeeze the lemons into the pot first, then drop them in the water. Stir, and turn the heat to high on the burner. Cover and bring to a boil.
Once the water is boiling, drop the potatoes and cook for 5 minutes. Add the corn, cook for another 5 minutes. Then add the sausage and mushrooms, cook another 5 minutes before adding the seafood. Make sure the water is back to boiling each time you add ingredients - should be a rapid boil.
The frozen seafood will drop the temperature, but as soon as it comes back to a boil, kill the heat and drop in as much ice as you can fit. Let it sit for 5-10 minutes, let drain another 5-10 minutes, then pour onto your tables lined with butcher paper (If using wood tables, best to add a layer of garbage bags or plastic of some kind to prevent water from leaking through and damaging the varnish).
Eat by hand, no utensils or plates. Serve with hot sauce, ketchup and cajun seasoning around the table - condiments can be poured directly on the paper. Ramekins of melted butter are good as well.