I've done a git clone into a directory, and normally I'd run "makepkg -si" for anything i'd got from the AUR. But this doesn't have a PKGBUILD, so i'm lost.
I've looked on the Arch Wiki and forum, but not sure how much it all applies to Garuda keeping track of what is installed, and where.
Is there a simple guide to tell me what I need to know?
As I checked your given repo, it has a makefile to install. So, First check if you have cmake installed or not, also as listed there running make all command from the directory will install the required dependencies. And at the end you need to do is execute the make install. This is the description. Now I guess your steps should be
sudo pacman -S cmake
cd "path where you cloned the repo"
ls */Just to check if you're in the correct directory"
make all
sudo make install
Paste the error logs in your reply if you get any.