Start with the reason this software should exist, not your preferred technology. What kind of user will use the system, how often, saas custom development and what does the process look like without it? A vendor who understands the goal will suggest a simpler way to reach it; a team that receives only a list of screens will price your assumptions along with the work.
Set out the scope as user stories or scenarios: a walk through each important path. Every bit as useful, list what you are not building. A written out-of-scope list prevents more friction at delivery time than almost anything else in the document. Also mark which items are decided and which may still change — the difference changes the price, and application support services hiding it only hurts you.
Write down the hard constraints. The list covers existing systems the software has to talk to, existing databases and their quality, regulatory obligations, traffic expectations, supported browsers or devices and infrastructure that is already decided. Where a date is genuinely fixed, say what depends on it: a team will often resequence the work to hit it, but only if they know it exists.
Define what the word done means for each item. Testable acceptance criteria need not use formal language: a plain-language note setting out the expected behaviour is sufficient. This single habit reduces acceptance testing considerably and removes most late-stage disagreement.
One last thing, say what you expect back. Ask for a task-level breakdown, the assumptions used, whatever the team considers risky and an optimistic and a pessimistic figure. Treat a wide range as a signal about the brief: it usually points to where your description is thin. Then tighten that section and ask for a new estimate — the next version is much more reliable.
