Start with the reason this igaming software should exist, not a feature list. What kind of user will use it day to day, how often, and what does the process look like without it? An estimator who knows what you are trying to achieve often proposes a cheaper route to it; someone handed only a list of screens can only price the list as written.
Describe 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 prevents more disagreement at delivery time than any other single page. Indicate as well which items are decided and which are still under discussion — honest teams price those differently, and pretending everything is fixed helps nobody.
List the constraints. This means existing systems the software has to talk to, the data you already hold and its condition, compliance requirements, user volumes, supported browsers or devices and stacks you cannot change. If there is a hard date, say why: an experienced team will often resequence the work to protect it, provided they hear about it early.
Write down what completion means feature by feature. Clear acceptance criteria need not use formal language: a short paragraph stating the expected behaviour is sufficient. This one section shortens acceptance testing by a surprising margin and edtech web application hosting removes the most common source of disputes.
Finally, state what you want in the response. Ask for a task-level breakdown, a written list of assumptions, the main risks and an optimistic and a pessimistic figure. Treat a wide range as a signal about the brief: it usually points to exactly which requirement is unclear. Then rewrite that part and ask again — the revised figure tends to be much more reliable.