Decide what size book you will be publishing. There are several different standard sizes for paperback and hardback books. Paperback books are typically 4 x 6 inches or 9 x 6 inches and hardbacks will typically be sized at 9 x 6 inches or 9 x 11 inches.
Calculate the width of the spine of the printed book. This will depend on several factors including the thickness of the paper used by the printing or publishing company as well as how many pages the book is. For example, a 200 page book printed on standard 50 offset paper would have a spine that is 0.391 inches thick.
Add in at least 1/4 inch of "bleed" space around the template. This is necessary to make sure that when the book cover is printed it is large enough and will not show any white space on any sides of the book.
Create a blank template in your design software. For example, for a 9 x 6 inch book, you would need to create a template that is sized for 9.5 x 6.5 inches at 300 dpi. The majority of printers work in 300 dpi and in order to make sure that your template is the proper size and resolution, it is necessary to work at this resolution. If you do not, the image will be warped when it is printed.
Mark off the space for the spine in the center of the template. Using your spine width calculations block off this amount of space in the exact center of your template. If you are using a full bleed image -- a book cover that has one large image that is used for the front and back of the cover -- set transparent markers to mark your space. If you are using a solid color for the spine simply create a block in that color in the size of the space you need and center it in the template.
Add in the other cover elements. The front and back cover design should be added, as well as the author name and title on the front of the book, the book title and last name of the author on the spine of the book, and a brief synopsis or reviews on the back. The ISBN number and bar code should be placed on the back cover approximately 1-1/2 inches from the bottom of the image.