Fuschia GO Park
Before you can do absolutely anything you'll need to connect your Pokémon GO account to your Nintendo Switch, which can be done as early as Fuschia City.
The Safari Zone is gone, replaced with the GO Park. If this is something you want to do immediately, then you can head to Fuschia City as soon as you have the Poké Flute from Lavender Town.
Once you have the flute you can head past the Snorlax - found South of Lavender Town or West of Celadon - and then go South to Fuschia City.
Linking your Nintendo Switch
Once you're at the GO Park you can go to the front counter and select "Bring Pokémon" which will activate the process of taking Pokémon from Pokémon GO, into the GO Park.
Once here go into Pokémon GO's settings menu and scroll to the bottom, where you'll find a Nintendo Switch option. You'll be able to link your Switch from here - both your phone and your Switch should be searching for one another, and your Switch should ask for confirmation.
Once the two are linked, you're ready to start filling up your GO Park. Go to your Pokémon menu on GO, and you'll find a Nintendo Switch icon in the top right. Tap that, and then start selecting mon to transfer.
One way system
The one thing you must keep in mind is that this is a one way deal. You can transfer Pokémon from GO to Let's Go, but not the other way.
That means this isn't an easy way to get legendary Pokémon on GO, and moving over any precious GO 'mons is a risky deal.
Be cautious and think long and hard about which Pokémon you want to transfer. You can transfer up to 50 Pokémon to each of the GO Parks, making this a nice way to clear space on GO without transferring all your Pokémon.
Catch 'em all
Right, now that the Pokémon are in the GO Park, it's time to catch 'em all, duh.
You can just stroll into the GO Park and all of your transferred Pokémon will be there. Just hanging out. It's perfect, you can waltz right up and, well, catch 'em.
Catching them works just like any other Pokémon in the game, and luckily, even if they run from your encounter, they'll still be waiting for you in the GO Park. Just remember to stock up on Poké Balls.