fantasai, a very prolific CSS spec editor, wrote a series of articles a couple years ago going into how the CSS WG works, in a lot of detail: fantasai.inkedblade.net/weblog/2011/inside-csswg
As to your specific question, typically someone suggests it, and if there are enough use cases and it's possible to implement performantly by browsers (not everything is, e.g. parent selectors), it gets accepted. This is where you, as a developer can help: If there is a feature you really want in CSS, collect as many use cases as you can and post in our repo: https://github.com/w3c/csswg-drafts/issues
If you make a good case, with enough evidence from around the Web that this is needed, it could get in!
However, keep in mind that even things that get accepted often don't really make it or take years. Getting something accepted is not enough, a spec editor needs to put the effort in to write the specification for this new feature, which is very hard work. So, if there's no editor that is interested enough, it could stall.