Describe the situation, not only the solution
Start with the problem: what is not working, who is affected and what the business wants to change. If you already have a solution in mind, describe it as a hypothesis. A good partner will test it, and may suggest a simpler or more suitable route.
What a useful brief contains
Business goal and the result you expect.
Users and the main tasks they need to complete.
Existing systems and data the solution must work with.
Constraints: budget range, deadline, regulations, preferred or excluded technologies.
How the project will be run: decision makers, contact person and review points.
What happens after launch: support, maintenance and further development.
Be open about priorities
Not everything can be first. Say what is essential, what is desirable and what can wait. This makes proposals easier to compare and avoids paying for features that were never needed.
Ask for a plan, not only a price
A proposal should describe the approach, stages, responsibilities and assumptions as well as the cost. Differences in price often come from different assumptions, so it helps to see them written down.
Agree how you will work together
Settle early how progress will be reported, how changes will be handled and how payments are linked to stages of delivery. Clear arrangements at the start keep the project on track and the relationship constructive.