Go to Saffron City
Enter the Silph Co. building
Defeat Team Rocket inside
Get the Silph Scope from the Rocket Hideout in Celadon City
Use the Silph Scope to clear Pokémon Tower in Lavender Town
Obtain the Poké Flute
Wake the Snorlax blocking Route 16
Go west through Route 16, Route 17, and Route 18
Enter Celadon City