A brief exists so the person doing the work can make good decisions without asking you every time. If it is longer than a page, it is probably a document nobody reads.
The objective
One sentence: what should be different after this exists. Not make a video, but help new customers understand how the service works in under two minutes. Objectives guide choices; tasks do not.
The audience
Who is this for, what do they already know, where will they encounter it, and what will they be doing at the time. Someone watching on a phone with the sound off needs different work from someone in headphones on a commute.
The deliverables
Be exact. Number of pieces, durations, formats, aspect ratios, file types, captions, language versions and where each one is published. Vagueness here becomes an argument later.
Tone and references
Give three adjectives and two or three references. Critically, say what you want from each reference. Sending a link and nothing else invites a guess. Also include one example you dislike and the reason, which is often more informative.
Constraints
Brand rules, legal requirements, things that must be included, things that must not appear, accessibility requirements, technical limits from the platform. List them plainly rather than mentioning them in feedback.
Budget and schedule
State a real budget or a range. Hiding it wastes everyone's time and usually results in a proposal you cannot afford. Give the deadline, the milestones and the dates when feedback will be provided.
Approvals
Name who gives feedback and who has the final word. The single most common cause of projects doubling in length is late input from someone who was not in the brief.
What to leave out
Leave out detailed instructions on technique. You are hiring judgement. Describe the outcome and the constraints, and let the specialist decide the method. If you truly need a specific method, say so and explain why.
Circulate it before work starts
Send the brief, ask for questions, and update it with the answers. A brief that has survived one round of questions is worth ten meetings.