Instructions
Go to the Cerulean Gym and find the missing machine part in the inner tube after defeating the Rocket Grunt. Talk to the director of the Power Plant and return the piece to him. He will fix the generator, restoring power to all of Kanto.
Travel to Lavender City and make your way to their Radio Tower. The man in the suit will reward you with a Pokegear expansion card that will upgrade your radio. You will now be able to listen to Kanto's radio channels. One of these channels is the Pokeflute station.
Heal your party at the Pokemon Center. Buy Ultra Balls from the PokeMart if you want to capture the Snorlax instead of defeating it. Make sure you have a Pokemon with sleep-inducing moves, such as Oddish, in your party.
Save your game in front of the Snorlax on Route 11 if you want to capture the Pokemon. Access your Pokegear and change it to the Pokeflute station. Talk to the Snorlax to awaken it, and the battle will begin.
Use sleep-inducing moves on the Snorlax. Once the Snorlax is asleep, lower the Pokemon's HP by using attacking moves. Whittle the Snorlax's HP to the red zone and use Ultra Balls to capture the Pokemon. Defeat the Snorlax if you don't want to catch it.