To get the Mew, follow the above steps to connect the Poke Ball Plus to your Switch. Then, open the Menu and go to Communicate then select the icon for Mystery Gifts

Normally, the Mystery Gifts menu only shows three options. If your Poke Ball Plus is connected, it'll show a new option allowing you to get a gift from the Poke Ball Plus. Select that to redeem your Mew.

The Mew comes at Lv. 1 with random stats (this will only show if you've earned the Judge feature). It will only know the move Pound.

Take care of Mew!