Well, just start, eventually everything will cascade into your mastery of the style sheets.

On a serious note; look at the CSS preprocessor projects, they are trying to address some of the pain points you are feeling. (https://www.keycdn.com/blog/sass-vs-less/)

I like reading spec documents when I spin at the gym: reading docs or books about a topic over and over again in parallel with actually using the technology in work helps me get a complete understanding.