When someone asked this question online the other day, I had to add my two cents to the pot. Here's my response.
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Simple. Start at the beginning and end at the end.
Next question, please.
What? Too simple? Okay, maybe it's not as easy as that. But, your question is pretty obtuse. If you're asking how I personally proofread and edit a book, that's one thing. But, since you have directed your question to more than one respondent, I'm assuming you're using "you" in the generic and not the specific sense. As in "How do you people proofread and edit a book?"
Honestly, anyone who sets about proofreading and editing a book, and doing so properly, needs years of technical knowledge about writing, language, punctuation, and spelling. Sentence structure also comes into play. As does syntax, characterization, dialogue, general structure, construction, and all the other elements involved in writing a fiction or nonfiction book (you don't specify in your question). After that, the editor must have a keen eye to identify a problem area when he or she comes across one and, of course, know how to fix it. There's no "one way" to do it right. But there often is one "best" or, rather, "most effective" way to do so, subject to personal preference.
If we can all agree with that assessment, then, we should all intuitively realize that few writers are qualified to proofread and edit books, particularly their own. It is not something that comes instinctively to anyone. This, despite most writers' mistaken belief that they're better equipped at writing and editing than they are.
In short, if you want to know how to proofread and edit properly, you search for the best, most qualified editors you can find, compare their rates and operating methods, and hire one.
By the way, I occasionally have a need to hire professional editors when I'm swamped with work and need some help to lighten my load. Despite my knowing virtually everything there is about editing and what to look for in an editor (from years of study, professional training, and on-the-job experience), even I get "taken" now and again: I'm often disappointed in the quality of the editing I receive. The same is true when my publishers assign their professional editors to my books. While some do a yeoman's job, none is perfect (or even close to it), which is disappointing. I often have to go back and correct an editor's work, explaining why my edits are preferable to (and more correct than) the professional's edits.
Still, hiring a professional editor (which includes proofreaders, by the way) always beats doing it yourself unless, of course, you also happen to be a talented professional editor, in which case it's …
Game, set, match.
Now, next question again, please.
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D. J. Herda is author of the new ebook series of writing advice, About Writing Right, available at amazon and at fine booksellers everywhere.