I can try to help. Are you using Linux or Windows? (I admittedly don’t have much experience using git on Windows)

Assuming you use Linux: usually, what I do is create a folder in my Documents directory specifically for handling Git projects (mostly because I like being organized), then open a terminal window there (right-click and press “Open Terminal Here”) or CD to its directory (for example, if it’s in home/<your username>/Documents/Git, run cd ~/Documents/Git).

Then, go to the github page, click the green Code button, and copy the URL there, which you will use to pull its git repository. Normally, you would then do git clone <git URL>, but the instructions say this uses submodules, so you should instead use git clone --recursive-submodules https://github.com/Mr-Wiseguy/N64Recomp.git. Don’t bother making a specific folder for this project because git automatically does that.

Then, make a folder inside the cloned git folder for containing the compiled build of the project (name it, say, “build”), and run cmake … (you may have to install this package first depending on if your distribution includes it or not) and then cmake --build. I think it then should be done.