When you have drawn a line with a pen or pencil along the ribbon or string marker from the previous step, use your craft knife or scissors to carefully cut your shape open along the line. If you have a balloon or a plastic bag shape as the basis for your box, it will be easy to slice through the layers of paper mache to cut the balloon and burst it, or to cut through the plastic beneath the paper mache without ruining the underlying shape. If you used a piece of china or a plastic egg or other container for your base shape, you will have to cut carefully through the paper mache, making sure you don't cut into the original you used as a basic shape. As long as your paper mache layer is dry, it should cut easily. This paper mache isn't usually more than 1/8 inch thick so you can cut it with sharp scissors or a craft knife.