Search the Internet with keywords such as "editor and fiction" or "editor and nonfiction," or post your job on elance.com or guru.com, giving the specifics of the project. Select a few candidates who offer details about their experience, preferably with book topics similar to yours. Some professionals specialize in copy editing, which corrects grammar and consistency, while others focus on content. Understand what you need from an editor.
Contact the editor and ask questions. You want to know what books they have edited before. Are they similar to yours? How many books is the editor working on now? Does the editor have contacts in the publishing industry?
Request a sample edit of one or two pages. You want to see the editor's comments on a piece of your own writing before you hire her. Don't hesitate to ask questions about the criticism. Are the comments productive?
Negotiate the price and deadline. Prices range from 20 to 40 dollars an hour. If the editor has worked on famous titles, the cost could be much higher.