In the main menu, go to the SPARRING section (the second from the top). You will see the character selection screen for a fight with a bot. To make a second player appear instead of a bot, connect a second controller. This can be done in several different ways:
- disconnect your Joy-Con controllers from the Switch and set them up as two separate controllers (by pressing SL+SR on each);
- bring additional Joy-Con controllers to the Switch and press the Sync button on the side of the controllers;
- bring a Pro Controller close the Switch and press the Sync button on the glossy top side;
- unsync any attached controllers by holding down the Sync button until the light does out. Then press any button on each one to set them as separate controllers.
In each of the listed methods, the controller will either be automatically registered by the game, or a special system window will appear in which you can assign a specific controller for each player.