In 5 steps :

1. Switch on the Switch™ console and select “Console settings” from the menu.
2. Then select “Airplane mode” and ensure that “Controller communication (Bluetooth)” is activated. If not, set it to “Yes”.
3. In the “Home” menu, select “Joystick” then “Change style/order”.
4. Hold down the “Mode” button (black Twin Pads) / the “Capture” and “Home” buttons (colored Twin Pads) on the right and left pads for 4 to 5 seconds. The 4 LEDs flash rapidly and the controller switches to synchronization mode.
5. repeatedly press the L trigger on the left joystick and the R trigger on the right joystick, as shown on the console display, until the pad(s) are detected.

Your pads are now synchronized and functional on your console: press button A as prompted on the console screen to exit the menu.

As a reminder, each pad can be used independently by using the SR and SL buttons on the edge instead of pressing the L/R triggers.