Yeah, all in the title.
How do people know how to skin stuff for 9.0/14, while the official documentation is outdated?

I've seen pieces of code with "AddBehaviour" for example, but how did people find this stuff?

Documentation tells you how to make the SkinInfo.xml file, and some other useful things, but it doesn't tell you how to modify messenger's ui code, you have to find how to do it. You can see other skins or ask to other skinners how to do something and probably they will tell you if they know.

P.D. documentation is not outdated.

I would have thought the first thing to do is come up with an idea...

If you are really new to skinning, then id probably do a skin for 8.5 based on Open Live so you get familiar with the style of coding etc.

But im not a skinner so i could be dead wrong
Good luck

Well, Im fairly experienced with Open Live for 8.5, but 9.0/14 uses many different new techniques, which I haven't seen in the documentation, such as AddBehaviour.

and, looking at other skins how they do it isnt what I want. I want to start from scratch - just as the skinners did.

The basic method is trial end error. You try different pieces of code and you wait for the result. That is the magic of skinning. Achieving something new after a lot of work.

I haven't yet started skinning on 2009. I mean I don't even know what changes were made. I just remember how I started back in 8.5