Open with the business problem, not your preferred technology. Which people will use the system, with what frequency, and what does the process look like without it? An estimator who understands the goal often proposes an alternative that costs less; someone handed only the requirements as given can only price the list as written.
Define what is included as user stories or scenarios: a walk through each important path. Just as important, list what you are not building. An explicit list of exclusions prevents more friction during acceptance than any other single page. Also mark which parts are firm and which may still change — estimators price uncertainty, and hiding it only hurts you.
Set out your constraints. The list covers systems you must integrate with, software development outsourcing uk the data you already hold and its condition, security and compliance rules, user volumes, target platforms and infrastructure that is already decided. If a deadline is real, php vs python say why: an experienced team is usually able to rearrange the plan to hit it, provided they hear about it early.
Write down what the word done means feature by feature. Acceptance criteria need not use any formal notation: a short paragraph describing the expected behaviour will do. That one addition compresses the review at the end by a surprising margin and closes off most late-stage disagreement.
One last thing, 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 useful information rather than evasion: it tells you exactly which requirement is unclear. From there tighten that section and request a revised number — the second estimate tends to be far closer to reality.