When I edit a book, this is how I work:
THE INITIAL EDIT: Before I read through the manuscript, one chapter at a time, I typically do my own set of preliminaries, which involves heavy use of the find/replace function and common error words. Believe it or not, most authors make the same types of mistakes to one degree or another, and getting the prelims out of the way right off the bat makes for an easier first-round edit.
I correct typos, misspellings, apostrophes, and misused words (there/they're/their; your/you're; to/two/too; then/than, etc.). Homophones and possessives are tougher for some people than for others, I gather.
I adjust phrasing in order to make something clearer. I add commas when it makes a difference. I am an unapologetic fan of the Oxford comma for clarity.
I will gently and lovingly stop you from writing, "Youth in Asia" when you really mean, "euthanasia." Yes, I've read things like this in published books, and it makes me laugh (or cry) every time. I once read a book in which the main character shouted, "Ludacris!" [an American rapper] when I'm fairly sure the author intended to use the word, "Ludicrous!" [when something is absolutely laughable]. The irony did not escape me there. That same book had a judge banging his "gabble," rather than a "gavel," as most other judges tend to prefer for courtroom use. I've edited books that have a "jester" instead of a "gesture," as well as other wrong word usage (emulate v. emanate, imply v. infer, to name but a few).
Unless you specify otherwise, I'll then approve all the non-negotiable changes, such as punctuation and capitalization, and make further adjustments if necessary. This is how I end up noticing the odd extra comma, the extra space between words, a lack of space between words, or something else that ruins the continuity of the read. If I've had questions or have made suggestions for rephrasing something, I will leave those approvals to the author. I don't change content or style without your knowledge.
I then send the manuscript back to the author for approval of the first round of edits. This is the opportunity to make any changes suggested or needed. If a manuscript is in need of heavy editing, I tend to work with frequent author contact because major, sometimes structural, adjustments are being made. Issues are dealt with right away, before further work is done.
FINAL COPY/FINAL EDIT: When the first round of edits has been approved, I reread through the manuscript again, checking for any changes made by the author, errors that may have been missed, and making sure certain words (adverbs seem to fall into this category) have not been overused throughout the book. If this is the last time my eyes will be on your book, I expect it to be formatted, with all front and end matter added, so you don't end up with an edited story but an error-filled acknowledgement page. Please include any promo blurb at this time.
The end result is a book that will show the author's hard work in its best light!
CONTENT EDITING/SUBSTANTIVE EDITING: I do not provide this service for full manuscripts. I will evaluate a partial manuscript to give you an idea of what's working or not, and will provide a written report (see my pricing page for details). If you're in need of a content edit for your entire work, which involves analysis of your manuscript, along with assistance and suggestions for plot structure, story idea, point of view, characterization, pacing, and writing style, I highly recommend Sarah at Your Beta Reader. Her prices are extremely reasonable, she's skilled, and if you mention I sent you, she'll give you a discount because she's nice like that.