Put simply, 'editing' can make or break your book. It is an absolute must if even a little part of you considers telling a story to someone. I use a handful of methods to help me clean up typos, sentence structure, and to remove excess words (though I still miss quite a few). I'll start with my least favorite, because it's also one of the best.
Reading the book backwards. That's right, reading it backwards. Have you ever tried that? It takes twice as long as reading it the correct way. Here's why: When reading a book naturally, our eyes skim over the flow of words because they're in recognizable patterns. We know how to string together a sentence. We can pick out the noun, verb, and direct object (if there is one). This ability lets us read an entire sentence at a time. It's the way we've been trained.
By reading the book backwards, a word at a time, the sentence loses recognizable aspects. It doesn't flow. It can't. Try reading this sentence backwards. Your brain slows down, forcing you to read one word at a time. And there's the benefit, the perfect way to check your spelling.
Another method is to print out your story, spread it out on the floor, and grab your pen. This way you can focus on an entire scene without having to scroll up and down, to worry about copying and pasting, or writing over what can be used later. Once you block off each scene, determine if it adds to the main plot or is filler. If a scene doesn't add to the plot, cut it out.
A third editing technique is to take each sentence in the story, remove it from the body of work, and rewrite it with an eye toward cutting out extraneous words. Extraneous words are words that are unneeded in a sentence. If you can tell the same thing in fewer words, go with that. Unfortunately, there is no hard and fast definition of what extraneous words are, but the story can be told without them, and no one will miss them. These Chains that Bind dropped from over 142,000 to under 130,000.
Extraneous word removal takes ALOT of practice. I've read These Chains that Bind seven times, and each time I cut out more words. I can only read two chapters before my brain decides it's done for the day. This way is exhausting but one of the best things you can do for your book. Tighter sentences means the book flows faster, making it a better read. A well-crafted sentence marks you as someone serious about writing.
Nothing highlights formulaic sentences, odd and dangling phrases, inconsistencies in description, and dozens of other problems like reading the book aloud. I'm serious. Listen for those moments that make you stop and have to reread the sentence. Something's wrong there; fix it. You can also hear when words a character normally wouldn't say come out of their mouths. Watch out for fomulaic sentences, like noun, verb, action; noun, verb, action; noun, verb, action. If you grow bored reading your own sentences, so will everyone else.
But one of the best things you can do before editing your book is to put it away for at least three months, maybe even a year. Don't touch it. Don't look it. Try to forget about it. The less you remember, the better. Nothing creates that objective eye like reading the story with fresh ones. Moments I thought worked, don't. Plotholes can be found this way, because now I'm the one wondering what's going to happen next. Sometimes this out-of-sight-out-of-mind perspective allows my subconscious to work on the plot, so when I come back to the story, I find new ways to fix old problems.
This last major editing technique is drastic and forces you to start the editing process all over again. I don't recommend it unless you are thoroughly unhappy with your story. I'm talking about rewriting the entire thing. I did that with These Chains that Bind. At that point, it had gone through three major revisitions, feelings of inadaquacies, unhappiness, and unsatisfacation before I got so desperate, I threw the whole book out and started all over again. I kept maybe 25 pages of the original 282 page book. The version I have now is its fifth, due to the word weight-loss.
Well, these are the major editing techniques I use consistently. But every writer has their own. Find the ones that work best for you and use them, because nothing makes a book shine like great editing.