Start with the reason this software should exist, not a feature list. Which people will use the system, with what frequency, and how is the job done today? A vendor who knows what you are trying to achieve will suggest a simpler way to reach it; a team that receives only a list of screens will price the list as written.
Define what is included as user stories flutter or react native scenarios: who does what, and what happens next. Just as important, write down what the first release deliberately excludes. An explicit exclusion list removes more friction at delivery time than any other single page. Mark too which parts are firm and which may still change — estimators price uncertainty, and pretending everything is fixed helps nobody.
Write down the hard constraints. These include the platforms and services involved, existing databases and their quality, security and compliance rules, expected load, target platforms and any technology you are committed to. If there is a hard date, explain what drives it: a team can often resequence the work to meet it, but not if the date is a secret.
Write down what completion means feature by feature. Clear acceptance criteria do not need special syntax: a plain-language note stating the expected behaviour will do. That one addition shortens the review at the end dramatically and eliminates the most common source of disputes.
To close, say what you expect back. Ask ai coding tools for development teams a breakdown by feature or module, a written list of assumptions, whatever the team considers risky and a low number and a high number. Treat a wide range as useful information rather than evasion: it usually points to where your description is thin. Then clarify that area and ask again — the revised figure will be the one worth planning around.