Open with the problem you are solving, freelance flutter developer not a list of screens. What kind of user will use it day to day, how often, and what does the process look like without it? An experienced team who understands the goal can propose an alternative that costs less; one who only sees a list of screens prices exactly what you asked for.
Define what is included as user stories or scenarios: a walk through each important path. Every bit as useful, write down what you are not building. A written out-of-scope list saves more disagreement at delivery time than the rest of the brief combined. Also mark which decisions are settled and which are still under discussion — the difference changes the price, and concealing the open questions helps nobody.
Set out your constraints. The list covers existing systems the articles on software outsourcing has to talk to, the data you have and where it lives, compliance requirements, traffic expectations, which devices matter and stacks you cannot change. If there is a hard date, say why: an experienced team is usually able to resequence the work to protect it, but only if they know it exists.
Write down what the word done means for each item. Testable acceptance criteria do not require any formal notation: a short paragraph setting out what must be true when the feature works is enough. That one addition shortens the sign-off process by a surprising margin and removes most late-stage disagreement.
Finally, ask for a specific format. Request an itemised estimate, the assumptions used, software development lifecycle stages the main risks and a low number and a high number. Read a wide range as a signal about the brief: it usually points to the part of the brief that needs work. Then tighten that section and ask again — the next version is far closer to reality.