How to Make Your Own Book Cover Template — Just the Basics
Determine the trim size of your book. For example a 6" x 9" book cover with a 1" spine will require a file that is, at minimum, 3975 pixels wide x 2775 pixels high set at 300ppi. Remembering that you have to include an 1/8 inch bleed on all sides, you calculate that number this way: (number of total inches wide for front, back, and spine + 1/4 inch total bleed) x 300ppi) x (number of total inches high + 1/4 inch bleed). So if your choice of trim size is 5.25" x 8" and you have a 1 1/4 inch spine and do the arithmatic, that winds up being 3600 pixels wide x 2475 pixels high.
Open your favored graphics program and set your file dimensions to what you've calculated that you need. Set the ppi to 300 and use sRGB or RGB set at 16 bit color. (32 bit color is for HDR images. You don't need it.) I'm not going to get into color or monitor settings any deeper than that because there are hoards of settings and details that will completely overwhelm a beginner, and, really, you usually don't need to worry about. Do understand that what you see on your screen without a calibrated monitor might differ from what another might see, so test your image on several different devices, and always print proofs of your paperback covers before hitting 'publish'. Many images must have adjustments to lighten them for print, because paper is not a screen. It doesn't have a lightbulb lighting it from behind. :)
Having set your image dimensions, now you need to set your guides. Set your template guides at 1/8" all the way around. That usually means setting four guides. 1/8" = 37.5 pixels at 300ppi, so, because you can't split pixels, for your outer bleed guide, set it at 38 pixels, then, for your 1/8" margin just inside that (another four guides), set those at 37 pixels inside the bleed guides. Set your spine fold guides (2 guides) (You'll need to do the arithmatic.), and set margins to either side of each fold (another four guides).
That's it. You're ready to go.
If you would prefer pre-made templates, I offer templates with help files for the most popular book cover sizes here.