There are some general rules that every style guide should have:
- Include enough to build a website from it - with the styles, examples and color palettes you provide, one should be able to create a complete website or mockup.
- The style guide must have examples - I've saw several style guides with great explanation and clear purpose, but the lack sample codes made them unusable. Add consumer ready code and your colleagues will love you...forever!
- Make sure the style guide is easy to understand - a newcomer or an intern should be comfortable using and understanding the style guide.
- Organize your style guide and stick to it - try to keep the style guide organized and clear.
Here is a wonderful website with all kind of resources about style guides - styleguides.io