Carefully! It also largely depends how it was treated before cooking. Some people cut through the head before cooking as a perceived "humane" killing method.
Assuming the lobster is whole, lay it flat on its belly, on a steady, hard surface. Use a stout, rigid bladed knife - this is very important. Place the knife at the back of the head, the sharp side of the blade facing forward. Essentially, hammer the knife down through the lobster with the heel of your hand and cut forward. Turn the lobster around and use the knife facing the reverse way to cut backwards to the end of the tail. This halves the lobster.
Pull off the claws. There is a special tool for cracking the claws but if this is not available, hold the claw firmly in your weaker hand on the surface and smack the knife down hard on the edge of the claw, well away from your fingers. Twisting the knife gently will crack the claw and allow it to be opened by hand.
The meat can now be scooped out of the two lobster halves and the claws.