It’s good news and bad news when it comes to using your Poké Ball Plus in Pokémon Sword & Shield.
We’ll get the bad news out of the way first. No, you can’t use the Poké Ball Plus to control your character in Pokémon Sword & Shield as you could in Pokémon Let’s Go, Pikachu & Eevee. It just doesn’t work, so don’t try.
The good news is you CAN use it to store a Galar Region Pokémon and take it for a stroll. You’ll be able to hear your little monster inside there by interacting with the control stick, and it’ll pick up little items for you from time to time.
But more bad news: you're stored creature won't passively gain experience from the steps you take. But they will grab random Pokéballs, Stardust, Rare Candies, and other sundry items while you're walking around the real world.
To connect Sword & Shield with your Poké Ball Plus, go to the menu and select "Mystery Gift," then scroll down to "Take a Stroll with Poké Ball Plus." Then hit either the top button or the control stick button until it connects with the Switch. Once connected, just follow the on-screen instructions to transfer over a Pokémon.
Keep in mind that if you forgot about an old Let’s Go Pokémon that’s still inside, you won’t be able to transfer it over to Sword & Shield. If you transfer a Galar Region ‘Mon to the Poké Ball Plus, it’ll overwrite whatever creature you have inside and eliminate whatever rewards they were holding. You’ve been warned.
The final piece of good news is that the Poké Ball Plus can net you a Mew for Sword & Shield. Every new Poké Ball Plus has a Mew that can be registered to one game and one game only. If you’ve already got a Poké Ball Plus and claimed your Mew, you can’t transfer it to Sword & Shield. But if you go out and buy a new one, you can claim it in Sword & Shield to have your very own Mew in Galar.