Create two separate files, one for the cover design and one for your book's interior. Start by choosing your book's dimensions, or how large you'd like the finished product to be.
Focus on the interior of your book first, as the cover design may change. Open your favorite piece of software such as Microsoft Word or Adobe InDesign to start working on your layout. Make sure you've edited your book, and then choose your dimensions or the size of your paper. Use a 6x9-inch or 5x8-inch layout for your interior to give your book the feel of a trade paperback.
Add a one-quarter to one-half-inch margin all around the pages to keep the text from getting cut off by the printer. Add another one-quarter to three-quarters of an inch to the gutter, or where all of your pages will come together in the finished piece. This is so readers won't have to open the book flat on a table in order to read the innermost text. Save the file and make a note of how many pages your document is.
Check with your printer to see how thick each page of your book will be. This measurement will differ depending on the printer you choose to work with as well as the kind of paper you use. You will need to know this measurement when creating your cover file for the spine.
Create a new file within an art program such as Adobe Photoshop or Corel's Paintshop Photo Pro. Begin designing your cover by taking your book's dimensions, adding another 0.125 inches to all sides if your printer uses a full bleed effect such as Amazon's Createspace. Add on the measurements of your spine based off the paper you have chosen for the interior and your printer's unique measurements found out in the step above.
Use the following formula to get an idea of how your cover may look.
A book that's 6x9 inches means your cover file, excluding the spine, will be 12.25x9.25 inches.
To estimate your book's design, you'd take the 12.25-inch width, add on the measurements of your spine based off the number of pages, and then use a height of 9.25 inches. Keep in mind that these measurements are taken from Amazon's "Createspace." Check with your printer before using these dimensions.