Turn on your Nintendo Switch
Open the Controllers menu from the Home screen
Select Change Grip/Order
Press and hold the Sync button on the controller until the lights start flashing
Wait for the controller to appear on the screen
Press L and R on the controller if prompted
Repeat for any additional controllers
For a Pro Controller, connect it with a USB cable or pair it wirelessly using the Sync button
For Joy-Con controllers, attach them to the console or pair them through the Change Grip/Order screen