It's a pain. Go to extensions, then nos2x, then details, then open extension website.
Discussion
I've managed to get that far - it opens the GitHub page, scrolled down and saw what I assume is the next instruction;
1 & 2 are fine, then there's:
3 click on "Load unpackaged";
4 select the extension/ folder of this repository.
There's nothing on the screen that says "load unpackaged".
oh you're using the version from github. that's fine. if you go that route, you need to enable developer options to allow loading of unofficial extensions.
Think I might have to go back over everything and start again - when I searched the extensions via kiwi I must have clicked the wrong one...it's fun getting used to all this