Open with the reason this software should exist, not a feature list. Who will use this, how many times a day, software development consulting and what happens today? A vendor who understands the goal can propose a simpler way to reach it; a team that receives only a feature list will price exactly what you asked for.
Describe the scope as concrete flows: what the user does and what the system does in response. Equally important, state explicitly what you are not building. An explicit list of exclusions saves more disagreement during acceptance than almost anything else in the document. Also mark which decisions are settled and which are still under discussion — estimators price uncertainty, and livewire vs alpine js concealing the open questions only hurts you.
Write down the hard constraints. These include the platforms and services involved, the data you have and where it lives, compliance requirements, traffic expectations, target platforms and stacks you cannot change. Where a date is genuinely fixed, explain what drives it: a team can often resequence the work to protect it, but not if the date is a secret.
Write down what the word done means for the important items. Testable acceptance criteria do not require special syntax: a short list setting out what must be true when the feature works is enough. This one section reduces the review at the end by a surprising margin and removes the usual argument at handover.
One last thing, say what you expect back. Request an itemised estimate, a written list of assumptions, the main risks and an optimistic and a pessimistic figure. Read a wide range as useful information rather than evasion: it usually points to where your description is thin. Then tighten that section and ask again — the revised figure will be far closer to reality.