Put the Joy-Con into pairing mode by pressing the small sync button on the side until the lights start flashing
On Switch 2, open Controllers
Select Change Grip/Order or Pair New Controller
Wait for the Joy-Con to appear
Press any button on the Joy-Con to complete pairing
Repeat for the other Joy-Con
If needed, connect the Joy-Con to Switch 2 with a USB cable or the Joy-Con charging grip and try again
If they do not appear, fully charge the Joy-Con and retry pairing