1. Convert to RTF (program varies depending on original file type).
2. Open RTF in Word, run spell check, set paragraph styles (body, chapter title, etc), add page breaks at end of chapters.
3. Convert RTF to epub with Calibre.
4. Open epub in Sigil and set the font size, line and paragraph spacing to my preferred standard. (could be done without Sigil, but you get a quicker preview of any changes with that).
But if your publisher is asking for a source file, they probably mean a normal word processor document they can play with themselves, not a finished epub file.