The main purpose of a book cover is to sell a book! Book covers are a marketing tool, so the right look is essential. Its design should inform as well as attract the reader. To compete with other self-published books on the market, good cover design is paramount. It requires eye-catching imagery; should be genre specific; use a good choice of colour and contrast; have legible typography, with a well-written blurb (the text on the back cover). I recommend using the services of a professional graphic designer. Although there are free-to-use cover design tools available online, these are limited in their scope. (Where an ISBN is allocated to the book, it will usually appear on the lower-right of the back cover, alongside the bar code.)
How I work. I contract out the book cover design to a colleague who is a professional graphic designer. The relevant page on her website can be found here. (Examples of her work can be viewed on her website, and on my Portfolio page: namely, Rupert's Diary, The Death Certificate, and the revised version of The Marriage Certificate.) She works directly with the client regarding the design and may produce several mock-ups of a cover before the final design is completed, and this is generally supplied as a PDF file. If an e-book is also produced, then the front of the cover is used as the display image.
How long will it take? Cover design generally starts at the same time as structural editing. A finished cover is required at the typesetting stage, in order to produce a proof copy of the printed book, but can still be edited until publication takes place. The cover will be bespoke: either photomontages or illustrative, and not made up of stock images to quickly create a design. The graphic designer guides the client through the process to achieve what the author wants, and the best route to market.
How much will it cost? The fee for cover design is agreed between client and graphic designer. Costs vary, depending on what needs to be done. For example, a fantasy or science fiction cover will be more expensive than a crime novel, simply because any imagery is likely to have to be created as an illustration; therefore, more time-consuming. For novels, cover artwork alone starts at £250.
What happens next? Ideally, book cover design should be complete by the time the manuscript has been typeset. Both are then ready to upload to the client's self-publishing platform, from where a printed proof copy of the book can be ordered by the client in readiness for it to be proofread.