If you're writing and self-publishing a book, there are a few must-haves, and a professionally designed cover is one of them, along with expert editing and formatting. As book publicists and other industry veterans can tell you, the competition is fierce, and you need to compete well with traditionally published books. The value of a well-designed cover is making an excellent first impression which means everything as media and book buyers are scanning shelves and web pages. While many people may have excellent graphic design talents, there is a specific art to cover design, and you want an experienced pro.
It's also worth mentioning that you'll do better if you have a custom-designed cover than using free art you can find online. Opt out of the cover design offer if you're buying a self-publishing package from an online resource. Too often, it means you'll end up with something that is not descriptive enough of your unique work. When you handle it separately, you'll have a cover designer who works with you directly, gives you suggestions, and listens to your feedback in arriving at the final design. For authors working with traditional publishers, ask to see samples if they will be assigning a designer.
To begin the cover design process, you can help tremendously by explaining your ideas about your book. If you hold back, hoping a blank slate will allow the designer to be more creative, there's a high probability they will come up with off-base suggestions. It's no comment on their talent or vision, but the reality is that reading your mind is tricky. If you give information that can be used as inspiration and a starting point, the process takes a more orderly path, and you'll spend more time refining a final design than struggling to find an initial one. Don't forget to compare the design against others in your genre.
Most cover designers will begin by giving you several options, and it's necessary to have faith in the process if you don't love the first batch. As you communicate more, it's easier to hone in on what you'll want ultimately. The first round may also help clarify your thinking because seeing what you don't like can lead you to specify things you like. Also, be open to the designer's ideas and recommendations because they have experience and expertise in the field. If you put too heavy of an imprint on the process, you may miss opportunities. Like the writing itself, cover design is a creative process.