This is a very inexpensive embellishment. You will need a piece of a cardboard box and a plastic floss pick as shown in the photo on the left.
Insert the pointed end of the pick between the first two ribs that are glued together. Shown in photo 2.
Holding the pick at a 45 degree angle between the already freed corrugations and the top layer of the cardboard, gently push the pick downward. (photo 3)
It may take you a few tries to find the angle and pressure that work best for you.