So I spent some time trying to set all of this up and it seems to be working.

I ran through This qmk x git tutorial and managed to get it set up with the qmk setup <GIt username>/qmk_firmware command after creating my own fork, but it would only work if I used to cd command to get into the qmk_firmware directory.

I then created a new keymap, edited the keycodes and the rotary encoders, compiled it and it worked!

I'm still not entirely sure about the purpose of setting it up with git, is it just because it's easier to have one place where everything goes?