Here's some things to keep in mind when preparing to hire a cover designer.
First, the price range. You can get a cover for $5 and you can get a cover for $500. I have personally paid both ends of that price spectrum. The old adage you get what you pay for tends to be true here. Someone getting $5 for a cover is probably going to give you a fairly generic result. The image is often a stock image that you could see show up on countless other authors' covers. If you are writing a spiritual children's book and you realize your cover looks exactly the same as a hard core erotic story, you might be less than pleased.
On the other hand, the idea of paying $500 for a cover for a book might seem quite high. However, a cover can absolutely make or break sales. And for $500 you are getting a unique, one-of-a-kind, custom model shoot or illustration. There is no way ANY cover will be like yours. You can create exactly the look you want. This is especially useful for historic and fantasy stories.
Make sure you do your research on the artist. Look at other covers they've done. Make sure they have a style you appreciate. Every artist has his or her own style and it's best to go with one that matches your desires.
Look at reviews from other authors. If there aren't any reviews posted, ask for a few recommendations. You want a sense of how willing the artist is to work with you on refining your image. A cover is rarely perfect at the beginning. It often needs tweaks and adjustments to match your mental image. Make sure the artist is ready for that process.
Also, make sure you know the time frame involved. Some artists have a quick turnaround and can work with you for a week until the image is just what you want. Others are juggling multiple projects and the task can stretch out to weeks or even months. Be prepared.
Your artist isn't going to read your entire book. It's a good idea for you to have a summary of the story and, if possible, a mockup of the type of cover you're thinking about.
Finally, make sure you're paying with a system that is secure. PayPal is often used. It's a good idea, if possible, to pay part up front and part on completion. That way you ensure the artist doesn't take your money and then never provide the cover.
To find an artist, first ask around with your author friends to get personal recommendations. Those are always best. If you don't find an artist that suits you, move on to sites like Fiverr.com and Freelancer.com. They will provide hundreds of options for you to sort through.
Make sure you are buying a commercial license to use the artwork. Some artists charge more for that. If you are distributing this with a book, it's a commercial product. Also, I highly recommend getting the source files (usually a Photoshop PSD file). That way if you need changes made later on you have that starting file to work with.
A last thought - be patient. Sometimes the first cover just doesn't work out. Creating a cover is a very tricky task. Many authors I know went through three or four artists before finding a combination that clicked. If the first attempt goes poorly, don't give up. Chalk it up as a learning experience and try again, knowing better what you want. You'll get there!
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