The main purpose of a book cover is to sell a book!
Its design should inform as well as attract a reader. Book covers are a marketing tool, so the right look is essential. For a self-published author wishing to be taken seriously, the book cover is as important as the other editorial processes. It requires eye-catching imagery; should be genre specific; use a good choice of colour, font, 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. There are free-to-use cover design tools available online, but these are limited in their scope. If an ISBN is allocated to the book, it will ultimately appear on the back cover, usually with a bar code.
How I work: When working on a self-publishing project, I contract out book cover design to a colleague who is a professional graphic designer. She creates the cover electronically, using specialised software. 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, and The Death Certificate.) I also recommend her for typesetting. Usually, she works directly with the client and may produce several mock-ups of a cover before the final design is completed. This will generally be supplied as a PDF file, ready for uploading to the client's self-publishing platform. The front of the cover becomes the display image for the e-book.
How long will it take? The timescale for cover design generally coincides with the first two editing processes, since the book cover is required at the typesetting stage in order to produce a proof copy of the printed book. Also, book covers are bespoke: either photomontages or illustrative. It's not simply a case of using stock images to quickly create a cover, but of guiding the client through the process to achieve what they want, and for the best route to market.
How much will it cost? The fee for cover design is agreed directly between the client and the graphic designer. Costs vary depending on what needs to be done, e.g. 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? The final cover design and typeset document are uploaded to the self-publishing platform; after which, a proof copy of the book can be printed. This is sent to the proofreader for the final stage of editing. See 3. Proofreading