Scoping starts with discovery sessions involving the people who actually run the process, not only the person buying the system. From those sessions you receive structured requirements to review and correct, then a scope of work setting out what will be built, what is excluded, the assumptions the estimate rests on, and an indicative timeline.
A written scope of work, structured requirements, a list of assumptions and exclusions, and a timeline. The assumptions and exclusions matter more than most buyers expect. An estimate is only meaningful alongside what it assumes, and a proposal without that section is not something you can hold anyone to.
Check what happens when an assumption turns out to be wrong. Check whether data migration is priced on the volume and quality you actually have. Check whether training and handover are included or listed separately. These are where implementation estimates usually drift, regardless of who wrote them.
You should come out of scoping able to explain the project to your own board, including what is not included. If you cannot, the scoping was not finished.
Bring your current process and your constraints to a first call. Book a scoping call.