Instructions
Obtain the Silph Scope by defeating Giovanni in the Rocket Game Corner. The Rocket Game Corner is the red-roofed building south of Celedon City's Pokemon Center. Inside, talk to the Rocket guard standing against the back wall and defeat him to gain access to the underground lair. Giovanni awaits at the end of the lair, and defeating him will reward you with the Silph Scope.
Defeat the Marowak ghost at the top of the Pokemon Tower. The Pokemon Tower is located in Lavender Town and holds a large collection of ghostly pokemon. The Marowak boss at the top can only be identified and fought if you possess the Silph Scope. Once you defeat the Marowak in battle, Mr. Fuji will appear and gift you the unique Poke Flute item.
Use the Poke Flute to awaken the Snorlax west of Celedon City. The Snorlax pokemon blocks your path to Fuchsia City and can only be defeated if you have the Poke Flute. Defeating or capturing the Snorlax in battle will remove it from the path, allowing you to enter the Cycling Road.
Travel along the Cycling Road to enter Fuchsia City. The long Cycling Road has a large amount of pokemon trainers waiting to challenge you, so be prepared for a series of grueling duels.
Defeat Koga, Fuchsia City's gym leader. Koga specializes in Poison-type Pokemon, which are weak compared to Psychic and Ground Pokemon. Once you progress through his gym and defeat Koga, the Silph Co. guard will move away from the front door, allowing you access.