Start with the reason this custom software development uk should exist, not a list of screens. Which people will use the system, how often, and what happens today? A vendor who grasps the purpose often proposes a cheaper route to it; a team that receives only a list of screens can only price exactly what you asked for.
Describe the scope as short scenarios: what the user does and what the system does in response. Equally important, state explicitly what you are not building. An explicit exclusion list prevents more argument at delivery time than almost anything else in the document. Also mark which decisions are settled and which are still under discussion — estimators price uncertainty, and hiding it helps no one.
List the constraints. These include the platforms and reactjs development services involved, the data you already hold and its condition, compliance requirements, fastify vs laravel expected load, which devices matter and startup mvp development infrastructure that is already decided. Where a date is genuinely fixed, say what depends on it: a good team can often resequence the work to meet it, provided they hear about it early.
Define what the word done means feature by feature. Testable acceptance criteria do not require special syntax: a short list setting out what a user should be able to do will do. This one section compresses the review at the end considerably and removes most late-stage disagreement.
One last thing, say what you expect back. Require a breakdown by feature or module, the assumptions used, the risks the team sees and a range rather than a single figure. Read a wide range as a signal about the brief: it usually points to where your description is thin. At that point clarify that area and request a revised number — the revised figure will be much more reliable.