Start with the business problem, not the deliverable
A clear creative brief begins with the challenge the business needs to solve, rather than a requested output such as a website, campaign or sales deck. Those deliverables may be appropriate, but they are not a strategy in themselves. The brief should articulate the direction for the work, the key insight behind it and the impact it is intended to achieve.
This foundation gives creative teams a defined problem to solve and a basis for judging whether concepts are relevant. It also makes the work accountable to something beyond personal preference. Strategy should remain present as concepts develop, acting as a reference point when an idea moves away from the agreed challenge or loses sight of the insight that prompted it.
Sources: Where logic meets magic: why strategy underpins every great idea, No creator is an island