Every design problem has its own set of peculiarities and requirements. Not only do you need to establish what these are but you have to prioritise them too. To hit the target, you can use a precision rifle or a scattergun - one spot-on solution or a number of solutions that are slightly differently focussed. Even doctors don't always know the exact way to treat an ailment, they may have to try a few alternatives - there's nothing wrong with that. But, some doctors are general practitioners and some are specialists. Their options and charges vary accordingly.
There is no magic formula for constructing a brief. As I said, every problem is uniquely different and needs to be treated as such, but here are a few basic guidelines to get you up and running.