Once the water has come to a full boil, insert the lobster. Here are some things to keep in mind: Submerge the lobster head first. Use the prongs to grab the lobster firmly behind its head and put it into the boiling water head first. This will numb it, so the lobster won't feel anything. If you put the lobster in tail first, there's a chance it can spread its claws and make things difficult, perhaps even splash you with the boiling water. Once it's fully submerged, put the cover on the pot.
Watch the clock. Once the water resumes boiling, let the lobster cook for about 12 minutes if it has a hard shell and weighs less than 2 pounds (roughly 1 kilogram). Add three minutes for each additional pound (1/2 kilogram) after that. For soft-shell lobsters, reduce boiling times by three minutes.
Rinse under cold water. One indicator that the lobster is done cooking is if it's bright red. Use the prongs to remove it from the pot immediately, and rinse it under cold water to stop the cooking process. When it has cooled down enough to handle, it's ready to eat.