A brand style guide does not need to be a hundred-page book. In fact, the longer and more formal it is, the less likely anyone is to open it. A useful guide answers the questions people actually ask when they make something for your brand: which logo do I use, which colours are correct, which fonts go together, and how should this sound? If it answers those clearly, it is doing its job.
Start with the brand you already have. Gather the logo files, colours, and fonts you use most often, and write down how they are currently used. If you are starting fresh, building a brand identity from scratch walks through the earlier decisions that a guide then records. The guide is not the place to redesign everything; it is the place to settle what has been decided, so the next person does not have to guess.
Start with the decisions people ask about
Put the most-used rules first. Logo usage comes before logo history. Show the primary logo, the version for dark backgrounds, and the version for small sizes, each with a short note about when to use it. List colours with their exact values, not just their names, because a colour described as “dark green” will be recreated differently by everyone who reads it. Name the fonts, say which one is for headings and which for body text, and show them at realistic sizes.
Keep the tone section just as practical. Instead of writing that your brand is “friendly and professional,” show a sentence in your voice and the same sentence written in a way you would never use. Examples settle arguments that adjectives never will. If a rule cannot be shown with an example, it is probably too vague to follow, so rewrite it or leave it out.
Keep it short, visual, and easy to find
A good test for a style guide is whether someone can find an answer in under a minute. Use pictures wherever a picture is clearer than a paragraph: correct and incorrect logo placement side by side, the colour palette as swatches, and a sample layout rather than a description of one. Most people will scan a guide, not read it from start to finish, so headings, examples, and short notes matter more than polished prose.
Treat the guide as a living document
Brands change, and a guide that never changes will slowly stop matching reality. When you add a new colour or retire a logo variation, update the guide at the same time and note the date, so people know which version is current. Settle the palette first — choosing brand colours with purpose covers that step — and then write it down. Outdated rules are worse than missing rules, because people follow them in good faith and produce work that looks wrong.