Designers use a brief to give them guidance and focus. A brief helps define the design problem and gives details on important considerations and constraints.
You might find it useful to develop your ideas visually before writing your brief.
Creating a spidergram is a good way to break down your aesthetic, functional and market considerations into key points. These can then form the structure of your brief.
The spidergram below shows how you might organise key considerations for an architectural design.