Open with the business problem, not a feature list. Which people will use it day to day, how often, and what does the process look like without it? An estimator symfony ecommerce framework who understands the goal will suggest an alternative that costs less; someone handed only a feature list will price exactly what you asked for.
Set out the scope as user stories or scenarios: a walk through each important path. Equally important, list what the first release deliberately excludes. An explicit list of exclusions saves more friction during acceptance than any other single page. Also mark which items are decided and which are still under discussion — the difference changes the price, and concealing the open questions helps no one.
Write down the hard constraints. The list covers existing systems the software has to talk to, the data you have and where it lives, security and compliance rules, traffic expectations, supported browsers or devices and stacks you cannot change. If there is a hard date, hire node.js coders say why: a good team is usually able to resequence the work to hit it, but not if the date is a secret.
Write down what the word done means for each item. Testable acceptance criteria do not need any formal notation: a short list describing what must be true when the feature works is enough. This single habit reduces the sign-off process considerably and closes off the usual argument at handover.
One last thing, say what you expect back. Ask for an itemised estimate, a written list of assumptions, whatever the team considers risky and a low number and a high number. Take a broad range as information, not evasion: it usually points to the part of the brief that needs work. At that point tighten that section and request a revised number — the second estimate tends to be far closer to reality.