Frankly, Smashwords can be just plain ugly unless you're good at fudging things or can work in native HTML fairly well. It has it's place, but I try to avoid it anymore and go straight to KDP or possibly CreateSpace if I need a printed copy (fewer and fewer these days).
But, the suggestion to use Word, especially if you're writing in it (I am) and follow the style guide for whichever service you use is quite valid. It's the simplest, for the non-technical crowd at least, to get a good final product. Yes, you can do marginally better with other tools and skills, but you're supposed to be a writer, not a pseudo-programmer.
My other advice is not to use anything related to Apple, but that's why about a third of this planet hates me. Or at least one of the reasons.
Bottom line in all of this is you have a learning curve, and it does get steeper before it levels out some. But if you don't keep learning, you die. Or that's what my third grade teacher once told me.
Jeff