Begin with a decision-ready brief
A good campaign brief makes the important choices visible. It explains the business context, the audience, the objective, the offer and the action you want people to take. It also records practical boundaries such as the agreed budget, the available assets and the people responsible for approvals.
The brief does not need to be long. It needs to be clear enough that strategy, content, design, production and account teams can work towards the same outcome. Unanswered questions are easier to resolve here than during a rushed final export.