Close editing, as distinguished from scanning-for-typos, is an intensely intimate enterprise. (Just to head off any misunderstandings. Really close editing requires not only seeing what's there, but seeing what isn't there and should be, or what is but is in the wrong place. It requires putting aside "how I would have said it" to be able to come up with something like "how you, at your best, might have said it."
From a blog post Title Editing and Intimacy
And a comment to post:
There are real editors in our midst and they can often be found lurking behind readable papers. They are professional editors. These are the ones who can transform muddiness and vagueness into spring water and clarity. They advocate for the reader. They suggest, improve, renovate, elucidate, spiff-up, improve (say, wait, does that need editing?) your documents. They turn snoozable material into a cup of good coffee. Professional editors may not only improve your writing but add perspective and light to your thinking.
There are organizations of them. There are good editors and not-so-good editors. There are content editors, and author's editors, and subject-matter editors. You can sometimes find them coaching students at universities.
Sure, some editing skills can be taught but editing requires a tremendous focus and discipline. Unless you're doing it full time, and can keep your skills sharp, you might just want to find a professional. And, when you find a good one, pay them fairly and keep them close.