Begin with the business problem, not a feature list. What kind of user will use the system, with what frequency, and what does the process look like without it? An estimator who understands the goal can propose a cheaper route how to evaluate software development company it; a team that receives only a list of screens prices exactly what you asked for.
Define what is included as short scenarios: what the user does and software architecture consulting what the system does in response. Equally important, state explicitly what you are not building. A written out-of-scope list prevents more disagreement during acceptance than almost anything else in the document. Also mark which items are decided and which may still change — estimators price uncertainty, and hiding it helps nobody.
Set out your constraints. This means existing systems the software has to talk to, the data you already hold and its condition, regulatory obligations, user volumes, which devices matter and any technology you are committed to. If there is a hard date, say what depends on it: a team is usually able to cut the right scope to hit it, provided they hear about it early.
Say what completion means for the important items. Clear acceptance criteria do not need formal language: a short list describing what a user should be able to do is sufficient. That one addition compresses the sign-off process dramatically and removes the usual argument at handover.
One last thing, ask for a specific format. Require an itemised estimate, the assumptions used, whatever the team considers risky and a range rather than a single figure. Treat a wide range as a signal about the brief: it usually points to exactly which requirement is unclear. At that point clarify that area and ask again — the revised figure tends to be much more reliable.
