Start with the reason this software should exist, not a list of screens. What kind of user will use it day to day, how many times a day, and what does the process look like without it? A vendor who understands the goal often proposes an alternative that costs less; one who only sees a feature list will price exactly what you asked for.
Describe the scope as short scenarios: a walk through each important path. Every bit as useful, write down what the first release deliberately excludes. An explicit list of exclusions prevents more argument during acceptance than the rest of the brief combined. Also mark which parts are firm and which are still open — the difference changes the price, and pretending everything is fixed helps nobody.
Set out your constraints. These include systems you must integrate with, existing databases and their quality, regulatory obligations, user volumes, supported browsers or devices and stacks you cannot change. If a deadline is real, say why: a team is usually able to resequence the work to hit it, but only if they know it exists.
Say what completion means feature by feature. Testable acceptance criteria need not use special syntax: a short paragraph stating what must be true when the feature works will do. This one section compresses acceptance testing considerably and removes the usual argument at handover.
One last thing, say what you expect back. Request a breakdown by feature or module, the assumptions behind each number, whatever the team considers risky and an optimistic and custom development vs saas ecommerce marketplace a pessimistic figure. Treat a wide range as information, not evasion: it usually points to exactly which requirement is unclear. At that point rewrite that part and nearshore software development ask for a new estimate — the second estimate tends to be the one worth planning around.