I won't say much about Pubit! because they do things right: I create an epub file in Apple's Pages program (Calibre also works, and it's cross-platform). I upload the epub file. Barnes & Noble publishes it. It's so simple and intuitive that no one would praise it, were it not for the alternatives. (I did have one problem with Pubit!: their accounting system failed to recognize my social security number, so I had to spend a little time with customer service, but that went fine.)
Though epub is becoming the industry standard ebook format, you can't submit epub files to Smashwords or KDP. You submit .doc files to them, and their converters create formatted ebooks. Alas, their converters can do odd things. The problem, if I understand it correctly, is that .doc files can hide invisible code which makes the KDP and Smashwords converters misbehave in ugly ways, creating bizarre formatting like misplaced italics and misaligned type.
My suspicion is that if you created an ebook in Word and never worked on it in another program, it would convert fairly easily at both KDP and Smashwords. But if your file has gone through several other programs, you're probably in for headaches.
What's extra frustrating is that while the process is similar for KDP and Smashwords, they have different quirks. I recently added a book to Amazon via KDP that made me think I'd finally mastered the process, only to find that the almost-identical file failed at Smashwords.
While I'm still learning, I'm ready to share what I've found useful:
PART 1: RESEARCH
Read the introductory material for Smashwords and Kindle Direct Publishing. The documention at both sites needs more work, but you got to start somewhere.
If you use Word, read Preparing to Smash Words and Smashwords Nuclear Code.
If you use Apple's Pages, read Creating ePub files with Pages.
PART 2: PREPPING YOUR .DOC FILE FOR SMASHWORDS AND KDP
1. Only use one font, or no more than two if you insist on a different font for titles. Use 12-point type for the main text and 18-point for titles.
2. Delete all tabs.
3. Replace two hyphens with a dash.
4. If you use # to designate scene breaks, replace it with * or •.
5. Replace underlining with italics.
6. Use smart quotes.
PART 3: WHEN YOUR FIRST ATTEMPT FAILS
Smashwords recommends a "nuclear option" which involves saving your file as a text file, then reformatting it. If you're as fond of italics as most writers I know, this is a horrible option.
The Word macro at Smashwords Nuclear Code may help you, or you may find it creates quirks. (It did odd things to a file with "#" symbols in it.)
Saving a .doc file as an .rtf file, then opening it again and reformatting the headings may help.
But if there's a guaranteed solution, I haven't found it. I'm going to see if I can convince a friend to make a program to strip everything from a .doc file except for the most basic formatting: justification, italics, bold, and underline.
To be continued!