Start with the problem you are solving, not a list of screens. What kind of user will use the system, react native software development company how often, and what does the process look like without it? A vendor who knows what you are trying to achieve can propose an alternative that costs less; one who only sees the requirements as given prices the list as written.
Set out the scope as short scenarios: what the user does and what the system does in response. Every bit as useful, list what you are not building. An explicit list of exclusions saves more disagreement during acceptance than almost anything else in the document. Also mark which decisions are settled and which are still open — the difference changes the price, and concealing the open questions only hurts you.
Set out your constraints. These include the platforms and services involved, the data you have and where it lives, regulatory obligations, expected load, supported browsers or devices and any technology you are committed to. If there is a hard date, say what depends on it: a good team is usually able to rearrange the plan to protect it, provided they hear about it early.
Define what done means for the important items. Testable acceptance criteria do not require any formal notation: a plain-language note stating what must be true when the feature works will do. This single habit reduces the review at the end by a surprising margin and removes the most common source of disputes.
To close, say what you expect back. Request an itemised estimate, the assumptions behind each number, the main risks and a range rather than a single figure. Take a broad range as a signal about the brief: best php development company it tells you exactly which requirement is unclear. Then tighten that section and ask for a new estimate — the next version will be much more reliable.