It’s actually really easy when you are not following the original manual.
GoPro Hero 8
Swipe down from the top, go to Preferences ➝ Connections ➝ Connect Device ➝ Smart Remote (yes, it works for the WiFi Remote too).
GoPro WiFi Remote
If it’s on, turn it off. Then press both of the buttons at the same time for about 1-2 seconds, which should trigger the pairing mode. Give it a few more seconds and it should pair with the GoPro:
(The remote manual says to hold the O button and then to press the power button, which didn’t trigger the pairing mode for me. When I pressed and held them down for a second or two at the same time, it did.)
I would say this will work also for the HERO9, as well as all the other GoPro’s since this remote is out since the old HERO2. Original manual to be found here.