The Show/Hide formatting symbols toggle button is a must whenever undertaking this process. However, you soon tire of seeing those little dots which show you extra space presses where you didn't want them. Gggrrrrrrrr!
If you haven't formatted yet, be WARNED. A strong coffee, and a lot of time is needed as you work through your manuscript. And a LOT of patience, especially if the place you are uploading to keeps returning your upload for revisions. TAKE YOUR TIME and get it right to begin with, it will save you much heartache, and headache come to think of it, when you are ready to publish.
If, like me, you use Word and are not aware of Styles, learn how to use them. Once they are set, they pay dividends in aiding the formatting process. You can make changes to the whole document, say chapter headings, in a few easy click through's, and if we're talking about a novel of hundreds of thousands of words long you can save a heck of a lot of time if you change your mind about a font size, type, or whether you want it BOLD or not, halfway through the formatting. And, once set up, you can change the main text of your story from one font to another, which is a great advantage if you want to create a difference between editions.
The following video is a great guide to getting started, by author India Drummond. Though, please be aware of Widow/Orpan control. Turning this function on will cause fluctuations in where your text ends on each page of your PRINT book, and can make it look untidy. If this is what you want by all means turn it on, I've since removed this feature from all my future manuscripts as I wasn't happy with the end result in my paperbacks.