Use the book that you wish to cover as your template. Open the book up and lay the cover flat against the paper. Draw around the book and then use a ruler to finish drawing your lines. Cut out all 4 corners and cut on each line that you will need to fold. Fold the book cover around the book. Make a spare and keep it on hand as a template for covering the same size book next time.
Buy a few book covers in the sizes you use most often. Store them away as templates for making your own book covers each time the need arises.
Visit your local library. Book covers are a common give away at libraries, they might have one that you can have for use as a template. If not, the librarian might allow you to use one of their plastic covers to trace onto paper and create your template.
Search for a book cover template on the Internet. Some sites offer free templates for an assortment of craft projects. Print the template and save it for future use.
Purchase photo computer software that includes templates for book covers.