There are a number of things to consider when choosing an editor, and I'll go through a few of them here, but the thing to remember is that all editors are different, just as all writers and manuscripts are different. Whilst you should be looking for a professional with the right credentials or background, you also need to find one that you can work with. After all, if you can't tolerate the briefest exchange with a person and don't trust a word that comes out of their mouth (or pen), you're not likely to create a written masterpiece with them, no matter how many qualifications and recommendations they have under their belt. So, while you're checking their social media profiles, consider whether or not you're going to like them enough to work with them. Consider whether or not they're likely to be the right fit for you.
Look for an Editor Who Works on Books Like Yours
There's no point in finding the most accomplished editor who ever existed, then sending off your science fiction novel, only to receive a response informing you that Mr Such-and-Such only edits cookery books. Look for an editor who works with your kind of book. Better still, find one who reads your kind of book too. It might sound odd, but not all editors read a lot (just as - and this is a fact that never ceases to amaze me - not all writers read a lot). If you have a horror novel that needs editing, look for an editor who reads the right kind of horror novels. Find out if they have a Goodreads profile, then go through their bookshelves. Check their social media accounts to see if they discuss their reading habits. Bookish people tend to post online about bookish things. Take me, for example; I'm all over the Internet... a veritable open book.
Look for Relevant Experience
Experience is good, but it has to be relevant. When an editor tells you they've enjoyed thirty years in the business, dig a little. 'Did you always edit romance novels?' you ask them. 'Well, in all honesty, no. I began doing that last Tuesday,' they reply. 'I used to edit books about gardening, and before that I worked for a scientific journal.' In the same way that no writer can write every kind of book, no editor can edit every kind.
Understand Your Own Requirements
Familiarise yourself with the various types of editing and choose the one that's right for you. You may need a proofreader to go through a finished manuscript to check for typos and punctuation errors just prior to publication. Alternatively, you may need a developmental editor to help with the plot, character development, dialogue, and to check for consistency. Some editors offer one service, while others offer multiple services, and it's best to make sure you understand exactly what is included in the service you choose before going ahead.
Some editors are more expensive than others, and the amount you pay will depend on the specific project; work within your budget. If hiring the editor you like most will leave you destitute, keep on looking until you find one you can afford. And some editors are booked up for months in advance. There's no point hiring an editor who can't begin work on your project for six months if you need it to be completed in a fortnight. Be honest and realistic about your requirements and expectations from the outset, and make sure that you and your editor are on the same page before commencing work.
Ask for a Sample
Most editors will offer a sample of their work to help you evaluate whether or not they are the right fit for you. Some may suggest sending you a sample from a previous editing job. The better option would be for them to produce a sample edit of your work: you send them a page or two from your project, and they edit it and return it to you. That way, you'll be able to gauge whether or not you like their editing style. Do their comments make you want to grab your pen (or laptop) and get right down to work on revisions without delay, or do they make you feel like rushing off to hide in a cupboard until you're ninety? If it's the latter, go back to your editor list and try out the next one on it.
Without wishing to make the process sound too mysterious, and beyond the understanding of all but the odd mystic who lives atop a snow-covered mountain, there is a certain, indefinable something that exists between a writer and editor when the mix is just right. You'll feel it in your bones (or your water) when you meet the right editor, as will he or she. They will get what you're trying to achieve, and you will recognise that fact.
One last thing before I go. The editing process can be a lengthy one - it depends on the project and how far you've already gone with it - and it can involve a lot of work, but it should also be an extremely rewarding experience. So, what are you waiting for. Go out there and find yourself an editor!