A look at the building first, because the answer depends on brick, plasterboard, distance and where people actually sit. This is why quoting WiFi over the phone does not work.
A written plan: what hardware, where it goes, what gets cabled, what it costs. If the honest answer is "move your router and change one setting", that is what you will be told, and it costs you an hour.
Installation, and then the part that gets skipped — testing coverage in every room rather than next to the access point, and locking the thing down. Admin passwords changed, firmware current, guest traffic separated from the machines that matter.
For businesses, this is normally where ongoing support starts making sense, because a network is only as good as the last time anyone looked at it.