Open with the problem you are solving, not a feature list. Who will use this, how many times a day, and how is the job done today? A vendor who knows what you are trying to achieve will suggest a simpler way to reach it; someone handed only a feature list prices exactly what you asked for.
Describe the scope as concrete flows: who does what, and what happens next. Equally important, state explicitly what you are not building. An explicit list of exclusions removes more disagreement later than almost anything else in the document. Also mark which parts are firm and which are still under discussion — the difference changes the price, and concealing the open questions helps nobody.
Set out your constraints. This means the platforms and services involved, existing databases and their quality, security and compliance rules, expected load, target platforms and stacks you cannot change. If a deadline is real, say why: a team can often resequence the work to meet it, but only if they know it exists.
Write down what completion means feature by feature. Acceptance criteria do not need formal language: a plain-language note stating the expected behaviour will do. This one section compresses acceptance testing considerably and node js development agency closes off most late-stage disagreement.
One last thing, say what you expect back. Request an itemised estimate, a written list of assumptions, 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 where your description is thin. At that point tighten that section and ask for native app development a new estimate — the revised figure will be much more reliable.