If you want to have some kind of structure for your css you should check SMACSS, OOCSS and BEM paradigms.
These are cool, and will give you proper direction if you want to write better CSS.
I'm not using SMACSS and BEM all the time because it's not always necessary but I like idea of OOCSS very much, so I feel like I always write modular CSS. Commenting might help too.
There is no answer for how to write it correctly, it's up to you to discover how you like to organize code :-)