Did you read the tutorial and manual? Both linked here:
https://www.mobileread.com/forums/sho...d.php?t=129364
If you have and can't figure out what to do we really can't help you without seeing a code snippet.
If you're saying conversion to pdf as an intermediate works then Heuristics will probably work, but the fact that heuristics will work would indicate that your headers aren't using h1/h2/h3 tags, which means something went wrong with the formatting in word and/or the conversion from word to html.