I have reservations about this approach. It sounds good superficially, but in real life, a brief is just one part of the equation unless you’re working in isolation and never interact with anything other than, say, an art director who hands you a brief, says “read this and do it,” then leaves.
Anyway, briefs run anywhere from some brief instructions in an email to a highly detailed, multi-page document.
It depends on the situation, I guess. If you’re a developer working overseas who receives nothing but detailed briefs from someone in another country, that’s one extreme. The other might be if you, the designer/developer, are working directly with the client or employer and negotiating with them about what they need and how much time it will take (and cost).
If you’re just working on refreshing your coding skills, I’m not sure why a brief would be helpful. As @Component suggested, just pick a small business’s website and tackle some aspect of making it better where you see a problem and where you’ll get to focus directly on polishing some of those skills that you feel needs the most work.
If you’re set on working from a brief, though, here’s one…