Begin with the problem you are solving, not a list of screens. Which people will use this, how often, and what happens today? An estimator best aso service who knows what you are trying to achieve can propose an alternative that costs less; someone handed only a list of screens prices your assumptions along with the work.
Define what is included as concrete flows: what the user does and what the system does in response. Just as important, write down what you are not building. A written out-of-scope list removes more argument later than almost anything else in the document. Also mark which parts are firm and which are still under discussion — honest teams price those differently, and hiding it only hurts you.
Set out your constraints. These include existing systems the software has to talk to, the data you already hold and its condition, compliance requirements, user volumes, target platforms and any technology you are committed to. If a deadline is real, say why: an experienced team can often rearrange the plan to hit it, but only if they know it exists.
Say what completion means for each item. Acceptance criteria do not need formal language: a plain-language note setting out what must be true when the feature works is sufficient. This one section compresses acceptance testing dramatically and eliminates the usual argument at handover.
Finally, hire expert react native app developers ask for a specific format. Require a breakdown by feature symfony or spring boot module, a written list of assumptions, the risks the team sees and a range rather than a single figure. Treat a wide range as useful information rather than evasion: it usually points to the part of the brief that needs work. From there tighten that section and request a revised number — the second estimate is much more reliable.