Cut one edge of the box and open it up flat on a flat surface. Measure the costume wearer from side to side across the front and front to back along their side. Measure the dimensions for your costume wearer on the box and mark the lines for folds with a yard stick and marker. The sides should be slightly bigger than the measurements of the costume wearer for comfort. Use the template (below) for a guideline, but be sure to cut the sides deep enough for it to be comfortable. Cut the box with a craft knife or box cutter along the lines.
Cut a stand alone piece from the cardboard to match the side measurement, adding 2 inches to the width on both sides. Fold the cardboard over at 2 inches on both sides creating flaps which will be used to attach the piece to the book. Paint this piece with white craft paint on one side. When the paint is dry, use a yard stick or straight edge to draw many lines from the top to the bottom of the piece with a fine tip black marker to create the pages. Attach the pages to the inside edge of the book cover with a hot glue gun, making it even with the opposite side.
Cut a shape from craft foam for the title of the book. Attach the shape to the cover of the book with a glue stick. Add adhesive letters to the title shape to spell out the title of your book. Cut four strips of foam about 1 inch wide and as long as the width of the sides. Attach two of the strips to the top and bottom of the spine of the book with a glue stick, pressing firmly.