Amazon.com provide a useful Simplified Formatting Guide to create an eBook using a word processor. The instructions are intended for using Microsoft Word to create a Kindle eBook, but are much the same as those from Lulu for creating ePub ebooks. The instructions discuss using the MobiPocket Creator application, but Amazon's Kindle Direct Publishing service supports converting a Microsoft Word document to Kindle format without use of any extra software.
Curiously the Kindle instructions say to use the older .DOC format, not not the newer DOCX (.docx). The latter is an XML based format, closer to the HTML which the Kindle uses. I found the Open Office ODT format (which is also XML based) worked better than .DOC for ePub eBook conversion with Lulu. So it is surprising DOCX does not work as well. I tried a DOCX file but found the conversion process failed with a spurious error message "Conversion timed out. Please make sure the uploaded file is less than 50 MB" (the file I was converting was only 88 kbytes).
One issue is that the Lulu tool I used created a table of contents, whereas it appears that Amazon's process requires the table of contents to be part of the book file. The Calibre eBook tool can create the table of contents for the eBook.