Many dedicated graphics programs exist, but writers without a design background find the learning curve steep. Mac users who own the word-processing program Pages already have an easy tool to make an ebook cover themselves.
First, have your art ready. Free and low cost art is available from many stock art sights. Try googling “stock photos”, “royalty free art” etc. Keep in mind that simple artwork without multiple complex elements work best for covers. Much of the art you will choose gets covered by text. Think background image.
In Pages open a document. Choose the “blank” template.
Under the File menu go to Page Setup … > Paper Size.
From the dropdown menu choose “Manage Custom Sizes.” Create a new custom size. Call it “Book Cover Ratio, set the width to 6 inches and the height to 9 inches. Leave the other fields alone and hit “Okay.” This will give you a page size the correct portions of a standard book, which will look professional when displayed on Amazon or other sites.
Drag your art or photograph onto your open document. Resize the art on the page by clicking and dragging any of the photo’s corners or edges. Use the “Lock” icon on the tool bar (or under the “Arrange” menu) to lock your art down and keep it from moving.
Under the “Insert” dropdown menu choose “Text”. This creates a box to type into. Create three text boxes: one for your title, one for your byline, and one for your cover blurb if you want one. (A cover blurb can be anything from a subtitle for the book, a teaser about the contents, or a one-phrase background on the author.) Separate boxes work best, so that you size your various text elements differently. Place the title text box near the top of your cover, and the byline text box near the bottom. Keep the blurb near your title, as seen on most professionally produced book covers.
Choose “Fonts” from the main toolbar, which will bring up the fonts window. Play with the fonts and sizes until a combination grabs you. Strive to keep your text readable. A single font works best. Avoid “distressed” fonts, or anything else difficult to read. For text color, black or white usually prove the most readable options.
With ebooks, most prospective buyers see a version of the cover at thumbnail size. Choose View > Show Page Thumbnails, which will allow you to judge how readable your cover appears.
Pages has no capability to save a document as a .JPG. Instead, save your document as a .PDF and then use iPhoto or another photo/graphics program to save the .PDF as a .JPG.
You now have a professional looking cover for an ebook that you can use with any ebook format.