Make a list of your specific needs as relates to the editing of your piece of work. Note things like what type of literature it is, the extent of your editing needs (whether copy and/or content) and your budget.
Search for an editor online. Use keywords specific to the piece you want edited. "SEO article editor" or "fantasy novella editor" are specific and descriptive enough to give you narrower results than an umbrella search like, "editors."
Make a list of all editors you have found that seem to fit your needs. Note specifics about each one. Does the editor have his own website? What does he charge? What services does he offer? Avoid editors who represent a company if you are looking to work with a freelance person.
Narrow down your results to between three and five editors. Pick those who seemed to best present themselves in a manner consistent with how you want to be presented. Choose editors who seem to best suit your needs.
Contact your candidates. Remember that you are hiring the editor, so conduct your consultation as if it was an interview. Make sure your editor is available for the time period you need, ask about her general scope of work and check her references.
Make an educated decision to hire one editor. Based on your needs and the interview results, hire your editor.
Negotiate and draw up a contract for services. This contract needs to detail payment, scope of work, time frame allotted, contact between editor and writer and the general expectation of the outcome. Make sure you negotiate the contract to satisfy both parties before signing.