Since you probably do not follow editors’ forums, I’ll spill: There are a great many people who first decided they wanted to be editors, then set forth to learn the English language.
For the record, that is not the proper order.
A high degree of English proficiency in at least one dialect is the baseline expectation for an editor, which means having been a voracious reader for at least a couple of decades. If one has to go on editorial forums and ask about punctuation because one’s chosen style guide doesn’t dictate one’s every action, one evidently doesn’t know enough about the language to make those decisions oneself. That’s like a military platoon leader who doesn’t know basic small unit tactics outside a field manual, and is afraid to improvise under fire lest s/he break a rule.
Writers take harm by hiring a less than competent editor, or by hiring the wrong editor. I’m not the right editor for everyone or every situation; no one is.
How would I go about it, putting myself in the writer’s chair?
I would learn what editors do. An amazing percentage of writers do not understand that there are different editing modes with different objectives and requirements. In nearly every case, my first job is to explain my job to the prospective client. They come in thinking “editing is when you fix all the things and crush my soul, duh.”
I would be clear and realistic about my goals for my project. If it was meant to make money, I would develop some marketing strategy beyond “hope to get discovered without doing any actual work.” I would take a guess at the type of editing that might best help me with my goals. I would prepare to be told otherwise, but I’d at least give it some thought.
I would ignore all the gig-economy.com sites where people can just list themselves and be hired directly. I would talk to other writers, ask about their experiences. I would eavesdrop on the Facebook groups for editors. I would observe the state of the art, all the people who need a committee meeting and an emotional support group to know where to put a comma, who treat the interpretation of a Chicago Manual of Style passage like rabbinic scholars treat Talmudic passages. I would look for the people who answer the questions, and how they answer them. I would pick out a few that seemed knowledgeable, intelligent, and successful enough to share their knowledge.
Then I’d get in touch, one at a time, but at first I’d let the editor direct the process. This would not be me abrogating my right to decide; rather, it would be meant to show that I wasn’t a control freak, and to observe the editor’s screening method. I would want to decide whether I liked that method, whether I found it helpful and promising. I would not profess to know anything about editing, though I would at least have done some basic homework. I would wait to see how well the editor guided me to a wise course of action and cooperation that would take into account my concerns and goals, about which I would expect to have been asked.
I’d keep doing this until I found someone that completed a good team, that I could afford, and above all was a knowledge sharer rather than a knowledge hoarder. This distinction is of paramount importance. Successful and skilled people tend to signify their success and skill by sharing knowledge in a generous fashion. They are never afraid they will run out of wisdom because they know how much they know. They are concerned not with being paid for every tidbit, but with giving the maximum value and support for any form of payment.
It’s expensive enough. You might as well get someone good–and that’s how I’d go about it.