Instructions
Head to Lavender town, the location of Pokémon Tower.
Enter Pokémon Tower and battle your way up all seven levels, facing off with the minions of Team Rocket as you go.
Rescue Mr. Fuji, who is being held captive on the top floor of the tower. He will reward you with the Pokéflute.
Travel to Route 12 or Route 16, depending on how far into the game's main storyline you have progressed. These are the only two locations in "Pokémon Blue" that provide the opportunity to capture a Snorlax.
Place a fighting-type Pokémon at the top of your roster. Normal-type Pokémon are weak only to fighting-types.
Play the Pokéflute to wake Snorlax up. He will attack you, at which point you should use your fighting-type Pokémon to wear him down.
Toss a Pokéball at Snorlax when he is weak to capture him. Since there are only two Snorlaxes in the game, it is best to use a Master Ball or other powerful capturing device.