Cut along one of the seams of the bag until you reach the rectangular bottom of the bag. Then cut the entire rectangular bottom out of the bag.
Spread out the bag flat on a table, with the printed side facing up. Place your textbook in the middle of the bag so it is centered. The bag will be much wider than the book, but only a couple of inches taller.
Open the book. Draw a line across the bag at the top and bottom of the book. Remove the book. You will have one line across the top of the bag about 3 inches from the top and another across the bottom of the bag (again, about 3 inches from the bottom, depending upon the size of the textbook).
Fold the paper bag down on the top line and up on the bottom line. Your paper bag should be the exact height of the book, but still much wider.
Place the book back in the center of the bag and open it up again. Draw a vertical line on the bag to mark the right and left edges of the book. Remove the book and fold the bag on those lines.
Slide the front hard cover of the book into the folded left pocket, and tape it closed. Do not tape it to the book itself. Instead, tape it on the top and bottom of the folded part. Do the same with the back cover of the book. Use a marker to write your name and the name of the textbook on the front of the book. Decorate it however you wish.