Start with the business problem, not your preferred technology. Who will use this, how many times a day, and what does the process look like without it? A vendor who grasps the purpose will suggest a simpler way to reach it; one who only sees a feature list can only price exactly what you asked for.
Set out the scope as concrete flows: real estate software development company who does what, hire flutter developer and what happens next. Equally important, state explicitly what is out of scope. An explicit list of exclusions removes more argument later than the rest of the brief combined. Indicate as well which items are decided and which are still under discussion — honest teams price those differently, and hiding it helps no one.
Set out your constraints. These include the platforms and services involved, the data you have and where it lives, security and compliance rules, traffic expectations, which devices matter and infrastructure that is already decided. Where a date is genuinely fixed, say what depends on it: a good team can often resequence the work to hit it, laravel vs nodejs but not if the date is a secret.
Say what done means for the important items. Clear acceptance criteria do not need special syntax: a short list setting out what must be true when the feature works is sufficient. This single habit shortens acceptance testing by a surprising margin and closes off the usual argument at handover.
end to end project development close, say what you expect back. Ask for a task-level breakdown, a written list of assumptions, whatever the team considers risky and an optimistic and a pessimistic figure. Treat a wide range as a signal about the brief: it normally identifies the part of the brief that needs work. From there rewrite that part and request a revised number — the next version will be far closer to reality.