Begin with the business problem, not your preferred technology. Which people will use the system, how many times a day, and what does the process look like without it? An estimator who knows what you are trying to achieve will suggest a simpler way to reach it; a team that receives only the requirements as given prices the list as written.
Describe the scope as short scenarios: a walk through each important path. Just as important, write down what the first release deliberately excludes. A written out-of-scope list prevents more argument at delivery time than any other single page. Also mark which decisions are settled and which are still open — honest teams price those differently, and pretending everything is fixed only hurts you.
List the constraints. The list covers the platforms and services involved, the data you have and offshore vs nearshore outsourcing where it lives, security and compliance rules, user volumes, target platforms and stacks you cannot change. Where a date is genuinely fixed, say what depends on it: an experienced team can often resequence the work to hit it, but only if they know it exists.
Write down what done means for each item. Clear acceptance criteria do not need formal language: a plain-language note describing the expected behaviour is enough. That one addition reduces the sign-off process dramatically and eliminates the most common source of disputes.
To close, ask react native developers for hire a specific format. Request an itemised estimate, the assumptions used, the main risks and a range rather than a single figure. Take a broad range as information, not evasion: it tells you exactly which requirement is unclear. Then rewrite that part and ask for a new estimate — the next version will be far closer to reality.