Open with the reason this software should exist, not your preferred technology. Which people will use it day to day, how often, and what happens today? An estimator who grasps the purpose often proposes an alternative that costs less; one who only sees a list of screens can only price 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, write down what the first release deliberately excludes. A written out-of-scope list removes more friction later than the rest of the brief combined. Mark too which parts are firm and which are still open — estimators price uncertainty, and hiding it helps nobody.
List the constraints. The list covers the platforms and custom web development services involved, the data you have and where it lives, compliance requirements, expected load, supported browsers or devices and infrastructure that is already decided. Where a date is genuinely fixed, say what depends on it: a team will often resequence the work to meet it, but only if they know it exists.
Define what done means for each item. Acceptance criteria need not use special syntax: a plain-language note describing the expected behaviour will do. This one section compresses the sign-off process by a surprising margin and monolith or microservices eliminates the most common source of disputes.
One last thing, ask for a specific format. Ask for mobile app development services a task-level breakdown, the assumptions used, whatever the team considers risky and a low number and a high number. Treat a wide range as useful information rather than evasion: it tells you where your description is thin. At that point clarify that area and ask for a new estimate — the next version tends to be the one worth planning around.