First, take one of the earphones out of the charging case. Wait until it enters pairing mode (flashing RED/BLUE), then double click the multifunction button – the LED will turn pink, then turn red for 2 seconds, and then turn off. Put the earphone back in the charging case.
Now, do the same steps for the second earphone. (Make sure do each earphone one by one, do not take both of them out of the case).
After performing the factory reset, take both of the earphones out of the case. They will automatically pair with each other and the left one will flash RED/BLUE, which means they are ready to pair with your devices.