Fold the tail over so that its curled back against the body, hold it firmly over the end of the tail so that it cannot open/flap its tail and put it head first in to boiling water, hold it until it has died, otherwise they splash around and its horrible. You can freeze them to kill them also but that can take a really long time, some argue that its more humane than straight into boiling water... Up to you, one may be painful and quick, and the other slow and not so painful? Either way its going to end up, well you know the rest.
Good luck, its never a pleasent job, hopefully it wont taint the meal