Instructions
Defeat the Elite Four. The Elite Four reside in the Indigo Plateau.
Talk to Professor Elm in New Bark Town. He will give you the SS Ticket.
Fly to Olivine City and board the ship in the harbor.
Fight every trainer on the ship. A man on the lowest floor of the ship will ask you to find his friend. Talk to his friend who is sleeping on a bed in another room on the same floor. An elderly man will also ask you to find his granddaughter. She can be found in the Captain's Quarters. She will ask you to play Hide and Seek with her, and once you've done this the boat will arrive in Kanto.
Fight the Vermilion City gym leader. Vermilion City is where the boat arrives. Before you can get to Pewter City, you will have to fight several gym leaders in order.
Head north to Saffron City and defeat the gym leader Sabrina.
Travel west to Celadon City. Defeat the grass-type Pokemon gym here.
Travel southwest out of Celadon City to Fuchsia City. Defeat the poison-type gym leader.
Travel east out of Fuchsia City and follow the path north to Lavender Town. Lavender Town doesn't have a gym, so continue north. Follow the bend in the path to the west and it will lead you to Cerulean City. Defeat the water-type gym leader Misty.
Fly back to Lavender Town. Visit the radio tower and a man inside will give you the EXPN card.
Fly to Vermilion City. Take the path east out of town and find the Snorlax that blocks the path. Use the EXPN card you just received to play the Poke Flute Channel on your Pokegear. Examine the Snorlax and defeat or capture it.
Enter the cave just past where the Snorlax was sleeping. Walk through the cave and exit the other side.
Use the move Cut on the small tree blocking your path and you will be in Pewter City.