It's not hard, once you know how. It only takes an hour or so, once you know how - finding out how took me forever - so here are a couple of useful tips. The first concept to understand is that Word is not always your best friend. In fact it reminds me of Terry Pratchett Igor characters, shuffling around helpfully in the background, inserting bits of liver or eyeballs that it thinks you might like. This concept especially applies if you have written your book at more than one computer, and it is quadrupled if you used more than one version of Word. I think we've all had the experience of Word stubbornly altering our formatting while we sit there shouting, 'No! Leave it alone! I don't want a capital letter/double spacing/whatever it is Word thinks is needed there.' Word sticks in all kinds of formatting that it thinks you might like, and the extra formatting upsets the E-publishing machines, which are machines, and need telling what to do, and are easily confused.
The publisher Smashwords has a useful style guide that you can follow. Most of my info came from there. You should read it if you are going to format your own book - but here's an overview. The first thing you do is to find Notepad and open a new document. Then you go back to your precious book, select all (CTRL+A) copy all, and paste all into Notepad - this gives you a clean, unformatted text.
Next open a new Word document and turn off ALL the auto formatting, (go to the Office button then Word options) because you don't want Word putting any commands back into your text once you've formatted it. Paste your text from Notepad into the new Word document, and there you go. A nice clean book in Word with no gremlins, ready to format for Kindle or any other e-publishing format.
You have to learn how to use Styles (there are plenty of good tutorials on the internet) because the idea is that you set the style you need for chapter headings, ordinary text and first line indents, and then, to make sure it's consistent, you click on the style to apply it instead of formatting by hand. It's also quicker, once you have got your head around styles.
The most important tips for me were:
Line breaks and page breaks throw up weird floating spaces. Reduce the distance between chapter ends etc to 1 space only.
If you use the tab command to make new paragraphs- that also makes the text in e-books float around, and readers complain. It's easy to fix: enter find ^t and leave the replace box blank and thousands of replacements will be made in two seconds.
The good news is that you don't need page numbers or anything fancy - the E-publishing machines will pop in all that is needed, except a table of contents, that technique eludes me so far.
Once you kno whow, It takes about an hour to scan through and get a document ready, but it's so worth the effort, as you'll see when you preview your text.