Use a ratcheting socket wrench with an extender and a 5/8" spark plug socket to remove the existing spark plug from the cylinder head. Make sure that the spark plug is completely seated in the rubber end of the socket before loosening it.

Spark plug sockets have rubber in the end of them to grip the spark plug and are made long enough to keep from snapping the spark plug. There are other common methods for removing spark plugs that involve using a long piece of rubber tubing to pull them out, but using a spark plug socket is the correct procedure.