Start by gathering all your ingredients and equipment. You'll need a large pot for boiling and a skillet for the sauce. Cut two of your lemons into quarters and save the third lemon for garnish. Slice the sausage into 1-inch pieces, halve the potatoes, cut the corn into 4-inch pieces, and make sure your seafood is clean.
Fill your large pot with enough water to cover all ingredients, about 2-3 liters. Add the quartered lemons, Old Bay seasoning, smashed garlic cloves, and onion pieces to the water. Turn the heat to high and bring everything to a boil.
Once the water is boiling, add your potatoes and crab legs first. Let these cook for 8 minutes since they take the longest. After 8 minutes, add the corn pieces and cook for another 5 minutes.
Now add the shrimp and sausage pieces to the pot. These only need 2-3 minutes to cook. You'll know the shrimp is done when it turns pink and curls slightly. Don't overcook or the shrimp will become tough.
While everything is finishing in the pot, make your sauce. In a small bowl, mix together the Cajun seasoning, paprika, cayenne, and Louisiana hot sauce. In a skillet, melt the butter over medium heat, add the minced garlic, then stir in your seasoning mixture.
Save 1 cup of the cooking broth from the pot, then drain everything else. Place all your seafood, sausage, corn, and potatoes in a large serving dish or tray. Drizzle some of the reserved broth over everything, then pour your warm Cajun butter sauce on top.
Finish by sprinkling fresh chopped parsley over everything and arrange lemon wedges around the tray. Serve right away while hot, with extra sauce on the side for dipping.
If you're using a grill or oven instead, lay out four large pieces of foil and divide all ingredients equally between them. Pour the sauce over each portion and seal the foil packets tightly. Cook on a hot grill for 12-15 minutes, or in a 425°F oven for 15-17 minutes until everything is cooked through.
Remember to provide plenty of napkins, a bowl for shells, and some warm water with lemon for cleaning hands. Serve with rice if desired, and encourage everyone to dig in with their hands for the true seafood boil experience.