Step 2: Part 2, Step 1: Preparing the Cover
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.