I just downloaded the zip to try and make a loading screen, but to be honest I had no idea what I was doing...perhaps making something similar to CustomMainMenu, would help...or even just adding documentation?

I haven't put much time into this in the last year unfortunately, but this was one of the big TODO's for the 1.10.2 and up versions.

I've put a little 1.3.x (for 1.7.10 and 1.8.9) docs together below, and I'll probably clean them up a bit tomorrow.
That being said, here is a very basic description of what the JSON tags do: https://gist.github.com/AlexIIL/fe5a0b9c5bc8a6bed6e2d20a11e56b41

(In your config folder you should have a folder named "BetterLoadingScreen", and in there it should have 4 files: "config.cfg", "images.json", "preset one.json" and "preset two.json". The "images.json" file is where the config goes, preset one and two contain examples that can replace images.json for default-alternatives and other example)

Alright, will look at that now, thanks :)

Is there an ETA on proper documentation? I've been trying to figure out how this works for the last 2 days and without the proper documentation, it's proven to quite difficult.

Most likely never as I don't work on CLS anymore in favour of buildcraft.

However if you have a more specific question then I'd be happy to help - either if you open a new issue, or DM me from the buildcraft discord.