Take a rectangular box, and then seal the edges with the help of a tape from all the corners of the box.
Apply the glue over the box on all sides. Then, cover the boxes with the bright, different and plain colorful paper from all the sides.
Take a craft knife and cut the mouth section easily and carefully. Cut two, rectangular and curved shaped mouth, in the middle of the box and one a bit lower than other.
Mark and Cut variety of shapes to make eyes and nose from your left over paper stock.
For final look of monster box accessorize with funny antennas on the head, some colored paper swirls to make hairs, and green triangle spots on the body.