Step 2: Part 2, Step 1: Preparing the Cover
Take a hardcover book you think nobody will want to read and cut out the pages with a knife. The way you do this is by cutting at the end papers, just inside both covers of the book, and pulling all the pages out.
Save the page block for use as a cutting/painting board later.
Next, place your holder inside the book, and draw lines below it, to cut off excess book cardboard. I decided to use the book board rather than cardboard because it's much sturdier for the bulk.
Use an Xacto or other knife with a ruler to slowly but surely cut through the bottom of the book. You can use a power tool if you like, but I find this is the best method for getting a nice, straight, even cut.