You have to work out the code.
the easiest way to start learning is probably this:
Get a clean _rank.gsc
Then download a mod.
Note that some modders 'arrange' their code in a very messy fashion, which makes it very hard to read let alone learn from.
So use a mod that is somewhat organised. I suggest looking at azumikkel mods, or mine for instance.
Now open the _rank.gsc from this mod and read the code and try to 'understand' it.
It's all very logical one you get the hang of it.
You can now start putting your code in the clean gsc.
once you get the hang of it, you can look for in depth tutorials on functions and codes.
Good luck! Have fun