Most crab legs are pre-cooked and frozen instantly aboard the processing ship within minutes of being caught so are fresh and ready to eat. If they are red they are pre-cooked. Follow these directions for pre-cooked crab. You do not need to thaw them unless you plan to then eat them cold which is one of the better ways. I thaw them in cold water over ten to fifteen minutes and serve as is, or with cocktail or tartar sauce or even hot melted butter with fresh lemon or lime juice mixed in.
To serve them hot. Heat a big pot of slightly salted water and bring to a boil. You can also add sugar to the water to bring out more sweetness in the crab. Or you can add no salt, sugar or anything to the water. Depends upon your taste. You can also use homemade (Bay leaf, cayenne powder, garlic, salt, sugar, and any other spices you like) or store bought spicy crab boil mix in the water such as Zatarains or Chesapeake Bay crab boil mix. Throw the crab into the hot water and turn off the heat. Stir once or twice while letting the crab sit in the water for 5-10 minutes until the crab is hot. Drain and serve plain or with melted butter, lemon or lime, tartar sauce, cocktail sauce, or plain. Since the crab is pre-cooked all you need to do is heat them up. Over cooking them makes them tough and tasteless.
If the crab is alive or uncooked then boil then for 5 minutes and let sit in the hot water for a 2-3 mintues before draining and serving.