Most places only require an 1/8th of an inch bleed. So if you just expand your document size by .125" on all sides you'll be all set. Make sure though you expand your colors and images into the bleed area. The whole purpose of the bleed is so that when the cover gets trimmed you dont see a line of white if the trimming shifts. But at the same time you have to make sure that your text and important stuff stays in the safe zone.
Use this link to download a template to help. Depending on what program and how many panels you're doing, you can pick what one you need.