1. Draw multiple versions of an individual cover element. A skull may, at first glance seem like a skull, but there are lots of things to consider. Is it face forward ¾ profile view? How big should the sockets be? Is it cracked anywhere? Doing lots of versions will help you later on.
2. Keep all of your elements on separate layers and do not merge until you are done.
3. Arrange all elements in place. Then save a copy.
4. Switch out any elements for alternate versions. Try out several combinations and save any that you like.
5. Print out your top 3 to 5 options. Then walk away for a few minutes.
6. Look for similarities, differences and elements of one that can be used in the final draft.
7. Merge as much as you can without compromising the quality.
8. Save your final version and now you can merge your layers if you want.