Start with the problem you are solving, not a feature list. Which people will use it day to day, how many times a day, and what does the process look like without it? An estimator python vs php who grasps the purpose can propose an alternative that costs less; a team that receives only a list of screens can only price your assumptions along with the work.

Define what is included as short scenarios: who does what, and what happens next. Equally important, state explicitly what is out of scope. A written out-of-scope list saves more friction at delivery time than the rest of the brief combined. Indicate as well which items are decided and which are still open — honest teams price those differently, and hiding it helps nobody.

Write down the hard constraints. This means the platforms and vertical software development services involved, the data you have and where it lives, regulatory obligations, user volumes, which devices matter and infrastructure that is already decided. If a deadline is real, say what depends on it: an experienced team will often rearrange the plan to meet it, provided they hear about it early.

Say what done means for each item. Clear acceptance criteria need not use any formal notation: a short paragraph setting out what a user should be able to do is enough. This one section reduces the review at the end by a surprising margin and closes off the usual argument at handover.

Finally, ask for a specific format. Require an itemised estimate, 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 tells you where your description is thin. From there clarify that area and ask for a new estimate — the next version is much more reliable.