When starting from a Word document I often use two alternative roundabouts:
1. Opening the document in Atlantis Wordprocessor and using the
"save as Epub" option.
2. Opening it in OpenOffice (or LibreOffice) and converting it to Epub by means of the add-on
Writter2ePub (
http://extensions.services.openoffic...ct/Writer2ePub)
In both cases you got an ePub already formatted and the code much more cleaner than the HTML you get directly from MS Word. Then you can fine-tune and customize it as you want with Sigil in a easier way.