Hopefully you've already read my posts about getting ready for an editor, budgeting, and choosing the type of editing you need, as well as evaluating your manuscript and plan. All of these things come before you start researching editors.
Once you understand why you're hiring an editor and what you want them to do for your manuscript, you can begin looking for editors who can fill these needs. There are many places to find editors online or you can get recommendations from your local writing community. No matter where the editor's name comes from, you'll still want to do your own research to make sure the editor will be a good fit for you.
What type of editing do they do?
One of the downsides in an industry so steeped in tradition is a lack of consistent terminology. There are many types of editing and knowing the name may not truly reveal what the editing consists of in this situation. Make sure to read their descriptions of their services and explain your expectations and concerns during the initial courtship.
What is their editing style?
Are they commenting? Making in-text changes? Are they pointing out issues or providing solutions? Are they enforcing strict rules or adapting to your style? Are they only pointing out things that are strictly wrong or anything that could potentially be improved?
Some of these things will be provided in the descriptions of their services or shown in their portfolio, but others may be best determined by a sample edit. Some editors do sample edits for free and others charge a fee. Editors may also be willing to answer these questions directly if a sample edit isn't a possibility.
What is their field of expertise?
Some editors might be experts in certain genres or age ranges. Others may specialize in certain style guides, such as The Chicago Manual of Style, APA, MLA, etc.
Be wary of editors who say they can work on anything. It may be true for copyeditors who have studied multiple style guides, but I've never met a developmental editor who truly understood the current trends and expectations of all the genres and age ranges. There is no one universal way to structure a plot. As a fantasy editor, I find horror plot structures mystifying. Adult-focused editors may have no idea what is appropriate in a middle grade novel.
If you have trouble answering any of these questions, make sure to keep discussing your expectations until you and the editor are on the same page. You'll also want to make sure you understand the pricing, payment schedule, and editing timeline before any editing begins.
Most editors will not be very flexible on any of these points, so don't view these conversations as a negotiation. If the editor isn't a good fit, you'll most likely need to use a different editor. Luckily there are plenty of great editors out there right now. It may take a little work, but you should be able to find an editor who understands what you want for your book and can help you reach your own vision.