You can reach Fuchsia City after obtaining the Poké Flute, starting from either Route 12 or Route 16.
Explore Fuchsia City
Go into the southeast house in the city, then go north through the back door, to find a Super Lure. Then go back into the house and use the south exit.
Go to the north part of the city to find the Safari Zone. After a cut scene, you can explore the area.
Go to the southwest part of the Safari Zone area and talk to the guy next to the surfboard. There will be a cutscene, and you will learn the Secret Technique Sea Skim. You can check on water to surf around with your partner pokémon, or you can put Lapras in your party and take it out of its Poké Ball to ride around on Lapras instead.
Explore with Sea Skim
While riding around on the water, you can catch wild pokémon that only appear in water, so be sure to go back to areas that you visited before and try surfing on the water there to catch some new pokémon.
If you go back and surf in Route 12, you can reach an island that has TM24 X-Scissor on it.
If you go to Route 24, you can surf in the southwest water, go south toward Cerulean City, then go west, then north. There is a cave that you can't enter until you have become a Champion of the Pokémon League. Go west from there to reach a slope that leads to a PP Max.
If you go to Celadon City, you can surf across the small pond west of the Game Corner, and talk to the old man there to get TM26 Poison Jab.
Now that you can use Sea Skim, there is an optional area that you can visit. Go to Cerulean City, then go east to Route 10. Get in the water in the northeast part of the route, go east in the water, then south, and follow the water to the end. You will find the Power Plant. This is an optional area where you can catch pokémon and battle trainers. There is a rare pokémon at the end of it.
Continue Exploring Fuchsia City
Back in Fuchsia City, go into the northern building of the Safari Zone. This is the GO Park Complex. Go to the upper part of the building and talk to the guy behind the desk. You can get explanations of how to use the GO Park Complex to bring pokémon from Pokémon GO to Pokémon Let's Go. For more detailed instructions, check the Pokémon trading instructions page.
In the Fuchsia City Pokémon Center, there is a Tamer who will teach special moves to your partner pokémon.
There is a Gentleman in the Pokémon Center who will trade you an Alolan Marowak in exchange for a Marowak.
Go south from the Pokémon Center to discover Jessie and James. During a cut scene, you get Gold Teeth. Go back toward the Pokémon Center, then go east and jump off the ledge, then go east and into the house with the mailbox next to it. Talk to the old man there and you will give him the Gold Teeth. He will give you a Safari Set for your Clothing Trunk. Then he will teach you Secret Technique Strong Push. Use it to push the stone block that is next to the TV. Then talk to the Diglett in the hole to get a Nugget.
When you are ready, you can challenge the Fuchsia City Gym.