Unfortunately, I'm running Windows 11. So I couldn't follow this guide at all.
I figured it was out of date, but there should still be similarities - specifically, the styles. That's the most important. Especially keeping the eBook down to no more than three styles. If you're not sure what these are, the attached doc highlights the bar. There have been other discussions in Litopia regarding styles, so maybe search around here, too.
What styles do you need?
One for the copyright page (for eBooks, a separate style isn't necessary, but I'd suggest centering at least), one for headings, and a normal (one normal indent, one normal no-indent). If you can figure out how to create and update styles, this is the most important skill to start with.
Learn that and you've saved several hundred dollars.
The title doesn't need a style, as there's only one. You can. It's simple enough. If you choose to 'design' the title yourself, don't get too fancy, as most eBook readers can 'adapt' the font and size to suit (and font usage has a price if you go for a fancy one - which also applies to font usage on covers).
Copyright page (aka colophon) is usually centred, with a smaller font.
Normal indent has a small indent, no space before or after.
Normal no-indent has no indent, no space before or after - unless the book is done entirely in block (all left aligned, no indents anywhere, in which case, 6pnts before or after, not both).
Headings are usually Heading 2, unless it's a book for subheadings (in which case, Heading 1 for main headings, heading 2 for subheadings, etc.).
In eBooks, do not justify text. It will look justified for the reader, but if the original doc is justified, it messes with the conversion and the output can be quite horrible (and often unreadable).
In an eBook, do not use page numbers, or any header or footer text. If you're doing non-fiction, and have footnotes and such, that's a separate learning curve (and a nightmare, depending on how and where you want the reader to 'see' the notes).
That's the very least to know, the absolute basics.