A brief is not paperwork. It is the agreement about what "good" means, written down before anyone has an opinion to defend. Projects without one do not fail at the design stage — they fail at the approval stage.
Here is the shape I use. It fits on one page.
1. What is the business, in two sentences
What you sell and to whom. If a stranger cannot repeat it back, the design will not communicate it either.
2. What is this piece supposed to do
Not "look modern". Bring walk-ins to a new location. Make a twenty-year-old company look current. Get people to scan a QR code. One clear job, stated as an outcome.
3. Who sees it, and where
Age, location, language, and the physical or digital context. A mailer read at a kitchen table and a banner glanced at from a car need different amounts of text.
4. What must be included
Logo, phone number, address, licence numbers, legal text, QR code, offer. List it now. Mandatory content discovered late is what breaks layouts.
5. Three competitors, and what you think of them
Links or photos. Say what you like and what you do not. This is the fastest way to calibrate taste, far faster than adjectives.
6. Constraints
Existing brand colours, fonts, a logo that cannot change, printer specification, size, page count, budget.
7. Deliverables and dates
Exactly which files, and the real deadline — the date the printer or the developer needs them, not the date it would be nice to see something.
8. Who approves
One name. If it is a committee, name the person whose word ends the discussion.
9. How you will judge it
The question that saves the most arguments. If the answer is "when I like it", say so honestly — then you both know taste is the criterion and can talk about taste directly.
If the client will not fill it in
Most will not, and that is normal. So interview them for twenty minutes, write the brief yourself, and send it back for confirmation. Their approval of your summary is worth more than a form they rushed through anyway.