Most self-published authors seem to think they can handle editing on their own. That may be true in a small percent of cases, however almost everyone needs a little help in this department.
Ideally, you’ll want to hire a professional to edit your work. If you don’t and you miss major mistakes, people will likely tear you apart in reviews and your sales will suffer badly.
If you truly can’t afford professional editing services, you should at least get some beta readers to look over your book in exchange for a free copy. Just tell them to email you a list of any errors they find in facts, spelling, grammar, etc. This will catch a lot of mistakes you miss yourself.
Another tip that will help you when editing your own book is to print the book and use a highlighter to mark mistakes. It’s easier to catch mistakes in print than it is on a computer screen.
Also, try editing one sentence at a time and reading each sentence backwards. This way you are more likely to catch mistakes that you might skip over, because when you read a sentence with a mistake, your brain will sometimes automatically correct it.