Start with the business problem, not a feature list. What kind of user will use the system, native app development how many times a day, and what happens today? An estimator who grasps the purpose often proposes a simpler way to reach it; a team that receives only the requirements as given prices the list as written.
Set out the scope as short scenarios: who does what, and what happens next. Every bit as useful, write down what the first release deliberately excludes. A written out-of-scope list removes more disagreement at delivery time than almost anything else in the document. Also mark which decisions are settled and which are still under discussion — the difference changes the price, and hiding it helps no one.
List the constraints. These include existing systems the software has to talk to, the data you already hold and its condition, regulatory obligations, traffic expectations, difference between php and python which devices matter and any technology you are committed to. If a deadline is real, say what depends on it: a good team is usually able to rearrange the plan to hit it, provided they hear about it early.
Write down what completion means for each item. Testable acceptance criteria need not use formal language: a short list stating the expected behaviour will do. This single habit shortens the sign-off process dramatically and closes off the most common source of disputes.
One last thing, ask for a specific format. Ask for a task-level breakdown, the assumptions used, the risks the team sees and a range rather than a single figure. Read a wide range as useful information rather than evasion: it normally identifies the part of the brief that needs work. From there rewrite that part and ask for a new estimate — the revised figure is much more reliable.