Think of your cover like a jacket made to wrap around the pages of your book with a perfect fit. A perfect fit is based not only on the trim size of your book but also the number of pages in your interior and the weight of your interior stock.
The two most common programs for preparing a book cover are Adobe InDesign and Photoshop. The file you make from this software is called a native file. Once you have finished your cover preparation in the native file, you will need to save it as a PDF (Portable Document File). A PDF will lock everything in place so no movement or changes occur when the file is sent to the printer.