Begin with the business problem, not a list of screens. Which people will use the system, with what frequency, and what happens today? A vendor who knows what you are trying to achieve can propose an alternative that costs less; one who only sees the requirements as given prices your assumptions along with the work.
Set out the scope as concrete flows: who does what, and what happens next. Equally important, write down what the first release deliberately excludes. A written out-of-scope list removes more friction at delivery time than the rest of the brief combined. Also mark which items are decided and which are still open — honest teams price those differently, and concealing the open questions helps nobody.
Set out your constraints. This means systems you must integrate with, existing databases and their quality, regulatory obligations, traffic expectations, supported browsers or devices and any technology you are committed to. If a deadline is real, say why: an experienced team can often resequence the work to protect it, but not if the date is a secret.
Write down what completion means for the important items. Clear acceptance criteria do not need any formal notation: a short list describing what must be true when the feature works will do. That one addition compresses the sign-off process considerably and next.js vs laravel closes off most late-stage disagreement.
Finally, ask for a specific format. Require a breakdown by feature or module, the assumptions behind each number, fintech development company the risks the team sees and react native consulting services a low number and a high number. Take a broad range as a signal about the brief: it tells you exactly which requirement is unclear. At that point clarify that area and ask again — the revised figure will be far closer to reality.