You can assign controllers in system settings, but let's use the in-game option which is a little simple. You actually do this in the Character Select screen, the one before that in which we enabled motion controls. This time the little '+/-' is on the top right.
Before doing this make sure your Joy-Cons are detached. Once you've pressed + or - you get the setup screen. Tilt your Joy-Con horizontally and press its shoulder buttons together - 'SL' and 'SR'. Now your controls will work with that horizontal controller, which is perfect for motion controls and the wheel accessories that are available.
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