Begin with the business problem, not a feature list. Who will use it day to day, how many times a day, and what happens today? An estimator who knows what you are trying to achieve often proposes an alternative that costs less; someone handed only a list of screens will price the list as written.
Describe the scope as user stories or scenarios: a walk through each important path. Equally important, write down what is out of scope. A written out-of-scope list prevents more friction at delivery time than almost anything else in the document. Indicate as well which parts are firm and which are still under discussion — the difference changes the price, and concealing the open questions only hurts you.
Write down the hard constraints. These include existing systems the custom software development company has to talk to, the data you already hold and its condition, regulatory obligations, traffic expectations, which devices matter and stacks you cannot change. If a deadline is real, say what depends on it: an experienced team can often resequence the work to meet it, provided they hear about it early.
Say what the word done means feature by feature. Clear acceptance criteria need not use formal language: a short paragraph describing what must be true when the feature works is enough. That one addition compresses acceptance testing dramatically and eliminates most late-stage disagreement.
Finally, state what you want in the response. Ask for a breakdown by feature or module, the assumptions behind each number, the risks the team sees and a range rather than a single figure. Take a broad range as useful information rather than evasion: livewire or vue it normally identifies the part of the brief that needs work. At that point tighten that section and native app development ask for a new estimate — the next version tends to be much more reliable.