Adding a Pokemon to the Poke Ball Plus is simple, but the menu can be tricky to find. You'll find the option to add your Pokemon to a Poke Ball Plus in the save menu shown below.

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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.

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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.