Begin with the reason this software should exist, not a list of screens. Who will use it day to day, with what frequency, and how is the job done today? A vendor who knows what you are trying to achieve can propose an alternative that costs less; someone handed only the requirements as given will price the list as written.
Define what is included as user stories or scenarios: what the user does and what the system does in response. Every bit as useful, list what you are not building. A written out-of-scope list removes more disagreement later than any other single page. Also mark which decisions are settled and which are still open — estimators price uncertainty, and pretending everything is fixed only hurts you.
Write down the hard constraints. The list covers existing systems the software has to talk to, existing databases and their quality, regulatory obligations, expected load, which devices matter and stacks you cannot change. Where a date is genuinely fixed price software development, say why: a team can often cut the right scope to meet it, provided they hear about it early.
Write down what done means feature by feature. Testable acceptance criteria need not use special syntax: a short list stating what a user should be able to do is sufficient. That one addition compresses acceptance testing by a surprising margin and closes off the usual argument at handover.
Finally, hire flutter developers say what you expect back. Ask for a task-level breakdown, the assumptions behind each number, the risks the team sees and a low number and a high number. Read a wide range as a signal about the brief: it tells you the part of the brief that needs work. Then rewrite that part and request a revised number — the next version will be far closer to reality.