I read a lot of Indie books. (Yes, I am kind of masochistic that way.) In general, people who don't have some sort of state or top-tier private college/university education in writing should hire someone who does to help polish their book. Yes, this is expensive, but it's actually a lot cheaper and easier than in the olden days when you had to hire a vanity press. Even people with upper level education in writing need editors to help polish, they just don't need as much polish. Beta readers are okay to use in addition to professional help, but they should be used more for finding out if your book is mostly coherent and enjoyable.
I repeatedly find the same mistakes over-and-over in new indie author work, so I figured I would address some of them here to help those who, for whatever reason, feel they can't afford the education or hiring someone. These are not all inclusive and can never take the place of a professional.
1. Not re-reading what you've written in one setting. This one always blows my mind. This is something that is drilled into middle school age students as soon as they are taught to write: "Proofread" your work before you submit it to your teacher. Like many goofy teachers below the university level, they misuse the word "proofread," but that doesn't change the fact that only the most immature writers think they can put out a written work without giving it a second read through.
Most of my novels are 50,000-75,000 words. If I am not working on the same book every day, I always go back and reread the whole story before adding to it. It is unbelievably easy to either write things that you, yourself, do not understand the meaning of because you misworded it or to forget what you have already said in the story. I mean, if you can't stand to read your own work, why would you push it on others?
2. Excessively and awkwardly using literary devices. If you have used the word "like" more than once every 1000 words (about 4- 8,5" x 11" double-spaced pages) and you are not writing some sort of simile induced poetry, you are not truly writing descriptively but rather overusing a literary device in your prose. Similes are the most basic and overused of all the literary devices. Unless you are writing literary fiction, literary devices should be used sparingly.
3. Author's notes before the book. Many indie authors like to talk about themselves and their books. Author's notes at the end of a book, explaining what inspired the story and the connection you hope the reader will make to it, can add a nice personal connection for readers. Most of the author's notes that indie authors use, though, are placed at the beginning of the book and these authors use them to either explain the book almost to the point of apologizing for writing it. If you have to explain details about your book to your reader before s/he reads the book--there is a problem with your book. If you have to apologize for writing it, than either you are trying to target the wrong market (that will reject it) or maybe you shouldn't be publishing it at all? There is certainly art for art's sake, but artists, directors, and authors who make this kind of art could really care less if you like it or not. If you, the author, do not feel your book is justified without a lengthy explanation, it is probably one of those things that you can regulate to the back of your closet for your kids to ponder some day instead of releasing it to the general public.
4 Mislabeling the Preface, Prologue, Introduction, or Foreword (including misspelling any of them). Although Author's Notes don't really belong in the beginning of a book, there are several ways you can introduce the book without making an apology for it. A "Preface" is kind of like an author's note. It covers what inspired the book and how it came into being and usually ends by thanking the people who helped with it. (Just thanking people would be an "acknowledgement.") Whereas an Author's Note would focus on making a connection with the reader, A Preface would give more insight into the process and focus on praising those who helped the author. A Prologue is part of the story, usually backstory that the reader must know for the rest of the story to make sense. This is not just random backstory that the author finds interesting. Authors should know all the details of their characters' backstories--the readers do not need to know all this information in a dump at the beginning of the book. An Introduction tells the reader information that is necessary for reading the book, but that is not part of the backstory. If you have created a unique world, where let's say gravity is reversed, this isn't really "backstory" information but it is necessary or the reader is going to be confused when s/he begins reading. A Foreword is where someone who is not the author (but who is famous) talks about how great the book is and how it affected them. As such, Forewords should never be found in Indie books unless you happen to be on good terms with your next-door-neighbor, who also happens to be someone like Stephen King or Johanna Lindsay, and you are so close that you don't feel like a heel asking him/her to (1) read your book and (2) write a foreword for it. If your neighbor is truly famous enough to write a foreword for your book and actually impress your readers, then s/he is probably not going to be able to tell you "yes" or "no" immediately, regardless as to how many times you have saved his/her cat from falling in his/her pool, because they will have to consult their publicist or manager or whomever is guiding them on their public image before they help you with your book.
5. Excessive typos. There is absolutely no reason for this, even in an Indie book. A few typos--whatever. Excessive typos? MS Word and Google Docs have a spelling and grammar checker built in. Grammarly has a free spelling and grammar check that is a little more advanced. Not using at least one of these is just lazy.
Doubting what these are telling you and making changes to the programming (i.e. marking something as correct when it says you are not) when you have never had formal training and just "think" you are right, is just dumb arrogance. Once you have told these programs you want to accept errors, it is difficult to go in and fix that.
6. Not understanding what a paragraph is. As a general rule, sentences do not make good paragraphs. In dialogue, there may be a back and forth of one-sentence paragraphs, but in prose, repeated single sentence paragraphs should not be used. Most of the time, these single sentences can easily be combined with other single-sentence paragraphs around them because they are still on the same topic.
7. Writing in a foreign language. If you don't speak the language, don't write in it. I read several languages well enough that I am confident translating into English from them. I only translate into English because I am a native speaker and native speakers of target languages are best because we are pretty familiar with nuances and idioms. That said, I feel confident translating into French/Spanish/Latin/Italian for the brief instances they may be needed in a book. For German, I can go to my Mutti, who is, well, German. Most newbies tend to think they can use Google translate or AI and that's close enough. It isn't. there are some garbage translations I have seen--to the point I have no clue what the author is trying to say because they chose some convoluted way AI told him/her to say it.
Also important on this topic: If your reader isn't going to understand what is happening in the book without you putting translation notes in it-don't write in a foreign language. I defer to The Lord of the Rings for the best example of how to do this. Tolkien was obviously inventing several languages. He was a linguist so he could do that and do it correctly. There is a glossary you can use to translate everything because without it no one would be able to translate what was said (he was the only one who initially knew the language he had invented, after all). Real, modern languages don't need glossaries because they are easily looked up online. At the same time, readers could read Tolkien and never look up what the orcish or elvish or dwarvish was saying because they never missed out on the actual story by not knowing what those words meant in English. Now, I have seen even traditional publishers, or rather Amazon imprint publishers, add in a translation after using a foreign phrase. In this latter case, the translation was not necessary because the rest of the story allowed the reader to understand what was happening. Having the English translation pulls you out of the story more than having text like:
"I love you, my sweet," Emilio whispered in German.
Using a foreign language for authenticity when you don't really know what you are saying and when you feel you need to translate it immediately back to English in the text is poor authorship.
8. Not understanding formatting conventions or using them inconsistently. Print books have indents at the beginning of paragraphs. Online and e-books have spaces between paragraphs. Some newbies combine these, which is confusing and wastes space. Other newbies fail to be consistent in formatting and italicize things randomly, switch fonts without seeming reason or consistency, and improperly use things like bold face and all caps. Publishing companies have style guides for those areas of grammar that are flexible in correctness but that need to be consistent throughout a manuscript. It might help if you establish basic rules and write/type them out in your own style guide document. Then go through your manuscript after editing to ensure you have followed these rules consistently. (This is called proofreading.)
9. Not citing works or including quotes without permission. If you quote anything in a book--fiction or non-fiction--you must cite the source. There are some times in fiction where using a footnote is a better choice that directly citing the source in the text. However, directly quoting another source in fiction--especially from songs--is plagiarism unless you have written permission from the copyright owner of that quote, and you then explain that you obtained that permission in the copyright notice. In general, you can quote up to 500 verses from the Bible in your work, as long as they meet certain requirements and you cite which version of the Bible you are quoting as well as the book, chapter, and verse of the quote. As an indie author, it is generally best to just stay away from direct quotes.
10. Not understanding your target audience. Every book has a target audience. Usually, the people who are your target audience are those who enjoy reading the genre you have written in. This means that if you write a mystery and call it sci-fi, when it doesn't have any sci-fi elements, then you are going to make your target audience unhappy. I have definitely seen mislabeled books--some are occasionally even put out by traditional publishers in an effort to sell books, like 50 Shades of Grey, that are erotica to a bigger audience. By marketing 50 Shades of Grey as a romance instead of as erotica, it reached more people than it would have, because fewer people read erotica than they read romance. This, is extremely risky because in doing this, you have to have a big enough marketing campaign to get millions of books into readers hands before bad reviews start flowing in. In other words, the book isn't that great but you already sunk a bunch of money into it, so you have to create a massive marketing campaign to recoup your money before people start letting other people know how bad it is. Movie producers also do this--giving you a trailer that has only the best parts of the movie and then you get there and the movie is awful. It's a gamble, but if you are going to lose money anyway and if you are a big publishing or production company, it is a gamble you may take.
In general, indie authors do not have well-funded marketing budgets. Five bad reviews can be pretty difficult to overcome, and it is even more difficult to get a decent reimbursement for your work prior to those bad reviews going live.
Aside from passing erotica off as romance, the other area where authors regularly misread their audience is marketing something as a "Christian" book. Many people are "Christians" and don't seem to realize that their behavior and the things they are alright with are not actually okay when one reads the Bible. Evangelical Christians, who see the Bible as literal, make up 1/4 of the population of the U.S. and they have and extremely strong review system online and through churches. If you slap the "Christian" label on your book, and it doesn't meet the standards of what a Christian book is, people (including myself) will complain, and they will do it loudly.
Granted, in my case, I complain against all mislabeling. I recently read a "mystery" series that was actually a soap opera in book form. That's the quickest way to get a one-star review from me--don't understand who your audience is and market to one audience when your book belongs with another audience.
