Start with the reason this software should exist, not your preferred technology. Who will use this, how many times a day, and what does the process look like without it? A vendor who understands the goal often proposes an alternative that costs less; one who only sees a list of screens will price exactly what you asked for.
Define what is included as short scenarios: what the user does and what the system does in response. Just as important, list what you are not building. An explicit list of exclusions removes more disagreement at delivery time than almost anything else in the document. Also mark which parts are firm and which are still open — honest teams price those differently, and pretending everything is fixed helps nobody.
Set out your constraints. These include existing systems the software has to talk to, the data you have and where it lives, regulatory obligations, expected load, supported browsers or devices and any technology you are committed to. Where a date is genuinely fixed, say what depends on it: a team will often resequence the work to protect it, which is better symfony or spring boot but only if they know it exists.
Define what completion means feature by feature. Clear acceptance criteria need not use any formal notation: a short list describing the expected behaviour will do. This one section compresses the sign-off process considerably and eliminates the usual argument at handover.
To close, say what you expect back. Request a task-level breakdown, hire smm specialists the assumptions behind each number, whatever the team considers risky and vue vs react a range rather than a single figure. Read a wide range as a signal about the brief: it normally identifies exactly which requirement is unclear. From there rewrite that part and ask for a new estimate — the next version tends to be much more reliable.