All three of the major self publishing platforms (KDP, D2D and Smashwords) have formatting style guides, which you need to follow carefully if you plan on self publishing an ebook, assuming you use Word, and your end product is a Word document for uploading. It's the stuff that you don't see on the screen that will get you.
They aren't particularly daunting, and once you have them figured out , using them will be like second nature and much easier than trying to hack formatting for printing. Control indentation using the slide bar (absolutely no tab characters), and use styles - Normal, No Spacing for text, and Heading 1 for chapter headings. Do not try to use fancy fonts and formatting beyond italics. That is pretty much as complicated as it gets.
Well okay I lied slightly. That is true for KDP. For D2D, it's a little bit more complicated because you have to have three line breaks for every two in order to preserve section breaks. I usually prepare my primary manuscript to KDP standards, then do a global search and replace to add the extra line breaks for my D2D version.
Smashwords is straightforward in that respect (same as KDP) except the uploader cannot parse a Word table of contents, and you have to manually create one using bookmarks and references. But since D2D is redundant to Smashwords, I don't even bother porting my books through Smashwords anymore since virtually all my non-KDP sales are through D2D. Readers seem to prefer the D2D version of the same book.