
Select "Take your Pokemon for a stroll" in the save menu to initiate the process of adding a Pokemon to a Poke Ball Plus. Follow the directions to connect your Poke Ball Plus to the Switch.
Now it's time to choose the Pokemon you want to bring with you.

The first option allows you to bring your Partner Pokemon -- the Pikachu or Eevee. The second option allows you to bring a Pokemon from your Pokemon Box. Select the option you need and then your chosen Pokemon will be sent to the Poke Ball Plus.
Return your Pokemon back to Pokemon: Let's Go by going to the save menu and hitting the "Take your Pokemon for a stroll" option again. This time when you connect your Poke Ball Plus to the Switch, it'll give you the option to bring back the Pokemon in the Poke Ball Plus.