Instructions
Walk east toward Vermillion city.
Continue past Vermillion City, until you run into the giant sleeping Snorlax blocking the road.
Wake the Snorlax with the Pokeflute. It will now move out of your way, allowing you to descend into the Diglett Tunnel.
Exit Diglett Tunnel through the other side, and travel south to Virdian Forest.
Walk south through the forest until you reach Pallet Town. From Pallet, you can use a water Pokemon to surf to the south and reach Cinnabar Island. From Cinnabar, there is canal your water Pokemon can surf on that will take you first to Seafoam Island, and from there back to the main continent beyond the construction workers.