Six steps, in this order
Who you are. Where the answer goes, and who signs the thing off.
What has to get built. The kind of site, and what has to be different once it is live. This is the part I read twice.
Who it is for. The part most briefs skip. It decides how everything else is written.
Content and materials. Text and photographs are what make launches late, not code. Better to know now.
How it should look. Two or three links beat any list of adjectives.
The practical part. Dates, access, integrations, who approves. This is what turns a brief into a quote instead of a conversation.
Answer what you can. "I do not know yet" is a real answer and I would rather have it than a guess dressed up as a decision.