Start with the problem you are solving, not your preferred technology. Who will use it day to day, how often, and how is the job done today? An estimator who understands the goal often proposes a cheaper route to it; a team that receives only a list of screens will price your assumptions along with the work.
Describe the scope as concrete flows: a walk through each important path. Equally important, state explicitly what the first release deliberately excludes. An explicit list of exclusions removes more disagreement at delivery time than the rest of the brief combined. Also mark which parts are firm and which may still change — honest teams price those differently, and concealing the open questions helps nobody.
Write down the hard constraints. This means the platforms and services involved, existing databases and web based software agency their quality, compliance requirements, user volumes, supported browsers or kotlin development services devices and infrastructure that is already decided. If a deadline is real, explain what drives it: a good team is usually able to resequence the work to protect it, but not if the date is a secret.
Write down what done means feature by feature. Testable acceptance criteria need not use formal language: a short paragraph stating what must be true when the feature works will do. That one addition shortens the sign-off process considerably and removes the usual argument at handover.
Finally, ask for a specific format. Request a breakdown by feature or module, a written list of assumptions, the main risks and a low number and a high number. Treat a wide range as a signal about the brief: it normally identifies the part of the brief that needs work. From there rewrite that part and ask for a new estimate — the next version will be much more reliable.