I went through the horrific process of self publishing Coffin Tales, so much that I thought I'd offer you a few tips and advice. (Turned out not to be so bad once I learned how to do it.) It's also the reason I'm submitting to agents and publishers. It's hard work.
If you want to go the Smashwords route, simply follow their style guide, not that hard to do. It may depend on how fancy your manuscript may be. Convert it to normal, single space and don't forget to indent the first line under format and then style, (paragraph). Take off your ruler, click on the invisibles (looks like a backwards P) use find and remove and take out all the extra spaces.
Use page breaks even though it looks funny in HTML on Smashwords. Use the center paragraph thing to center your title and chapters. Insert all the copyright information. If you're unsure what to use copy it from another ebook you bought, they pretty much all say the same stuff. Don't forget a dedication page and give credit where it's due, always mention the editor and cover artist.
Surprising how many people don't, that really pisses off an artist. It's bad enough they have to put up with writers, lol. Speaking of artists, remember to be clear what you want and communicate it properly the first time. Because if you don't, you may be expected to pay more. It's only fair as artists work hard and with the recent changes in publishing they are making less and less money all the time. Most covers are nothing more than a picture, yes, it's a sad day for artists.
If you are going the Amazon route, let me give you the biggest piece of advice I learned from my formatter. The preview you get on KDP is not right. I converted it to HTML on word, and it turned out great, even though it looked wrong on the KDP preview. It's one of those dirty secrets formatters don't want you to know. I also converted my document to normal for this process.
I think that about sums it up. Always, always edit your book before putting it up anywhere. I can't believe how many mistakes I made, but I had no formatting errors on Smashwords on my first try. (I did find plenty of editing errors though.) I also wasn't happy to hear many people had trouble trying to get the Kindle version from Smashwords. One of the reasons Coffin Tales Season Of Death is now free on Smashwords.