A primer to start writing CSS with BEM-style syntax by Johan Ronsse. Good stuff, although I disagree with Johan when he says BEM is unnecessary for small and personal sites.
Personal project or not, writing CSS with BEM makes class-naming much more constrained (and well-documented), pushing you to write styling and markup in a more structured manner.
If you’ve never heard of BEM, this article by CSS-Tricks is another great way to get started.