These are the supplies you'll need. I'm sure most of you already have these things on hand.
Begin by flipping your paper over to measure the front and back covers. I make a small pencil mark for the length and another one for the width. You want your paper to fit exactly to the edges of the composition book, with a very slight overlap on the black binding.
Now use those marks to cut your piece of paper. And imagine that I took a picture of this for you.