Open with the business problem, not your preferred technology. Which people will use the system, how often, and how is the job done today? A vendor who grasps the purpose can propose a cheaper route to it; one who only sees the requirements as given prices the list as written.
Set out the scope as concrete flows: a walk through each important path. Every bit as useful, list what is out of scope. A written out-of-scope list prevents more friction at delivery time than any other single page. Also mark which parts are firm and which may still change — the difference changes the price, and pretending everything is fixed helps nobody.
Write down the hard constraints. These include existing systems the software has to talk to, the data you already hold and its condition, compliance requirements, traffic expectations, target platforms and any technology you are committed to. If a deadline is custom real estate software development, say what depends on it: a team can often rearrange the plan to hit it, top node.js development company but not if the date is a secret.
Say what the word done means feature by feature. Clear acceptance criteria need not use special syntax: a plain-language note describing the expected behaviour is sufficient. That one addition shortens the review at the end by a surprising margin and closes off the most common source of disputes.
One last thing, blockchain development outsourcing ask for a specific format. Ask for an itemised estimate, the assumptions behind each number, the main risks and a low number and a high number. Treat a wide range as a signal about the brief: it tells you exactly which requirement is unclear. From there rewrite that part and ask for a new estimate — the second estimate will be the one worth planning around.