Whether you’re just starting out, or you’ve been writing for some time, it’s no secret that the most successful writers specialize in a niche. Why? Focusing on a niche helps you become an expert in that area, write better content, ask better questions, and know where to find sources and research. The best editors typically specialize in one or a few niches for similar reasons.
As a freelance editor, I would like to express my strong disagreement with the idea that a freelance editor must work as an employee of a publisher for three years to be any good. In my field, spirituality, many publishers have gone to having most editorial tasks beyond acquisitions performed by freelancers, so the in-house positions being held out here as a prerequisite for freelancing simply don’t exist; many editors must cut our teeth as freelancers. It also occurs to me that self-published authors usually bristle at the idea that they are not “real” authors unless their work is accepted by a commercial publishing house. Is it fair to apply this standard to the people they propose to employ as editors?
A good editor is invaluable when starting out. Later on it perhaps depends on how complex the story or writing is. Editors can be looking for similarity not originality. The final script a poor representation of the original story or article. An article of mine, which was picked up by Wikipedia for its originality and informative ideas was given forty edits from around the world! It was like a mechanical invention rather than an interesting article in the end!
When you’ve found someone who seems like a decent candidate so far, make first contact. (The contact form on the website is fine as a first step.) Introduce yourself and state what you are looking for (a developmental critique, copyediting, etc.). Give a very brief description of your project, including its genre and approximate length. Give an idea of your planned next step and any deadlines (even if only approximate ones). For example, have you already signed a contract with a subsidy press for publication next fall? Are you planning to submit the manuscript to publishers and/or literary agents after editing?
Regarding your last question, I know of no editor who gives a money-back guarantee for occasional errors that happen to slip through or for a different opinion by a later editor. I’m afraid such things are just the nature of the publishing beast. But by having a shorter portion edited first, you will get a decent feel for whether someone is the right editor for you, and will be able to be confident in his or her work on your entire book.