Certainly, to be an editor, you must be good at English or whatever language you’re editing. Also, you must care. Be a perfectionist. No one else involved in publishing a document, whether it’s a blog post or a book, may be attuned to the nuances of words and punctuation. The editor may have to compromise or be overruled, but striving for excellence is essential.
Also, pay attention to the details but don’t get mired in them. John McIntyre, copy editor for the Baltimore Sun, wrote on his You Don’t Say blog that some people in publishing think that copy editors are “comma jockeys”; he pointed out that by careful attention to content, copy editors had prevented the paper from publishing libel and plagiarized writing. Be aware of the content: is it coherent and complete? Also, unless someone else is doing the fact checking, I recommend verifying anything you can. This takes time and patience, but the alternative—getting names, numbers, and titles wrong—is worse.
One choice for editors is to specialize in certain subjects. For example, Katharine O’Moore Klopf, who maintains the Copyeditors’ Knowledge Base website, is a medical editor. Adrienne Montgomerie, a major contributor to Copyediting.com, edits scientific works.
Copyediting.com has a lot of articles to assist freelancers, and Liz Dexter’s Libro Editing blog has, besides lots of helpful information for editors, a continuing series of interviews with small business owners. Her book How I Survived My First Year of Full-Time Self-Employment: Going It Alone at 40 is available at Liz Broomfield Books. I reviewed it on my website.