Start with the reason this software should exist, not your preferred technology. Which people will use it day to day, with what frequency, and how is the job done today? A vendor best react native development company who knows what you are trying to achieve can propose a cheaper route to it; a team that receives only the requirements as given prices exactly what you asked for.

Define what is included as user stories or scenarios: a walk through each important path. Every bit as useful, list what is out of scope. A written out-of-scope list removes more argument during acceptance than the rest of the brief combined. Indicate as well which parts are firm and which may still change — the difference changes the price, monolith vs microservices and pretending everything is fixed helps nobody.

List the constraints. This means the platforms and services involved, existing databases and their quality, security and compliance rules, expected load, supported browsers or devices and stacks you cannot change. If there is a hard date, explain what drives it: a team is usually able to cut the right scope to protect it, but not if the date is a secret.

Say what done means feature by feature. Testable acceptance criteria need not use any formal notation: a short list stating what must be true when the feature works will do. This single habit shortens the sign-off process considerably and closes off most late-stage disagreement.

Finally, ask for a specific format. Ask for a task-level breakdown, the assumptions used, the main risks and an optimistic and a pessimistic figure. Read a wide range as information, not evasion: it normally identifies the part of the brief that needs work. Then tighten that section and ask again — the revised figure is much more reliable.