Line your ruler up on the diagonal end points of your paper and trace with a pencil. Then do the other side. You should have a big X when you are done.
Once you have made the X, cut up the lines to about 1/2" from the center. Do this on all sides.
Using hot glue, fold each of the corners forward and glue one at a time. (use the same side of the sections each time so the shape comes out right).