Instructions
Go to Route 11 and walk up to the sleeping Snorlax. It will be blocking the path to Diglett's Tunnel and Vermillion. You will also need the radio Expansion Card from the Radio Tower in the town of Lavender to wake it up.
Open the PokeGear and select the radio. Drag the cursor to the top of the center line on the radio dial. It will say "PokeFlute" when you reach the correct frequency.
Exit out of your inventory, but leave the radio on. Talk to the Snorlax with the "PokeFlute" frequency playing and it will wake up and fight you.