Here's a partial checklist of skills needed to successfully self-publish. Basically, you need to reproduce all the services publishers provide, only each of their departments are led by experts.
1. Great editing skills. If you're self-publishing, you need to hire a pro. Prices start around $8, $10 a page.
2. Eagle eye and a solid grasp of grammar and punctuation. You'll have to proofread and copy-edit your ms. before it gets published...or hire someone to do it.
3. Book design. How are you at graphics and book design? Books need to be visually appealing.
4. Artistry. Someone has to do the book cover and make it look professional. For best results, you need not just an artist but an art editor.
4. Tech know-how: It's not hard to upload a book to Amazon or other self-publishing venues, if you've done it before.
5. Marketing: Know the best way to reach appropriate readers? Know how to make your book stand out from the crowd? Know how to attract free publicity and create buzz? If not...oh well.
6. Publicity: Have lots of influential media friends, preferably some who can get your book discussed on tv and public radio.
7. Reviews. Have a contact list of mainstream reviewers who know you and will agree to review your book for their paper, even though mainstream media won't review s-p books.
8. Advertising: Not every published book gets an ad budget, but the big titles do. What's your budget?
9. Social media: Have a huge presence on your social media of choice: 20K is a reasonable number for starters. Must be real followers in order to convert to buyers.
If you can do all these things well, self-publishing may be for you! If not, maybe you should consider looking for a publisher, tough as that is.
I feel like this is meant to be very discouraging, and as someone who is a modestly-successful self-published author, yes, get your book edited and have a nice cover, but I can't say I agree with the advice overall.
One thing you have to ask yourself when you're self-publishing is what your goals are. What does success look like to you? Often self-publishing advice assumes certain goals which might not be relevant to many writers.
The list above seems to assume you are shooting for a very large number of sales. And that...isn't what everyone's goal is. I remember listening to a podcast and a romance author was interviewed. She was published with a trade publisher and talked about fearing a book would sell only 5000 copies. And I would LOVE if one of my SP books sold that much. It would make me a good chunk of revenue, too, since I make close to $3 per sale for my novel-length books.
The list is a list of things a trade pub would do...ideally. But, many writers get shoddy editing, limited marketing, etc. even with such a publisher, and the advantage of self-publishing is that you have control, and control means you get to decide where to focus your efforts.
Addressing some of the specific points: $8-10/page is high for edits, and I assume this would be for multiple rounds of developmental edits, plus copyedits. I only pay for one round of edits (not ideal, but I'm still reasonably happy with what I put out) at 1.5 cents a word, which is less than $4/page (assuming 250 words/page)
Cover art: I pay my cover artist $115 USD per cover. (Plus a little more to get Twitter ads, social media banners, etc) I choose the stock photos (my preference). We go back and forth a bunch of times to perfect it. People generally seem to like my covers. I have no idea what an art editor is.
Social media: I'm going to focus on Twitter, because I think Twitter has done more for me than anything else in terms of selling copies. I write romance. Most romance authors I know don't have 20k followers. They certainly didn't have that many before they started publishing--those numbers came later. I have only 2885 followers (just checked), and I STILL find that Twitter does quite a bit for me. From what I can see, actually, getting more than 10k followers as a romance author is often accompanied by a lot of trolling and bullying on Twitter, which can be quite annoying, so I don't particularly aspire to have that number of followers.
To me, the most things to consider are:
1) What are your financial resources and what are your skills?
2) What is your projected output (books/words per year)
3) What are your goals with self-publishing, and why are you self-publishing?
I've put out 13 books (novellas and short novels) in just over two years. I'm definitely not rich, but I was never trying to get rich. Among a certain part of the romance community, I am somewhat well-known, and I got an agent and book deal with a Big 5 publisher, (without a completed ms)...and the strength of my self-publishing career definitely helped here. I still plan to keep self-publishing (because I like it) and be a hybrid author. I can publish about 250,000 words a year. This year, that amounts to 3 short novels and 3 novellas. I think it's hard to have a single book achieve really amazing success in self-publishing, but I have a lot of books in me, and my income has built over time, and I get to tell the stories I want to tell.
One thing I do think is important is knowing a lot about your genre. And I don't mean reading the classics...I mean, what's currently selling, what kind of covers are people using, where do readers hang out online, etc. Also beware of taking advice from self-published authors who built up their audience 5-10 years ago...stuff changes rapidly, and what worked then doesn't work now.
For me, self-publishing was the best publishing decision I ever made, and I don't have much in the way of connections or marketing, art, etc skills. (My degree is in engineering.)