I've been teaching how to make this box for several years. It is the perfect size to hold a number of regular sized cards and envelopes. I promised a guest at last Saturday's workshop that I would post this on my blog... so here it is.
You simply need two sheets of cardstock: one for the top and one for the bottom. After stamping the top as desired, you need to score 2 1/2" in from each of the 8 1/2" sides and 2" in from each of the 11' sides. Then, make 4 cuts along the lines that come in from the 8 1/2 " side, just until they meet the perpendicular score line. You should have something like this:
Fold to form a box, folding the extra half inch over as shown below:
Adhere the flap with a strong adhesive such as sticky tape. Repeat for the box bottom adding about 1/16 " to the measurements for scoring. Thus you would use 2 1/16" and 2 9/16". By increasing these measurements slightly, the bottom of your box has a little smaller circumference and will be just slightly higher. This allows the top to fit on more easily.
If you have been around Stampin' Up for a while you might own a copy of Book III of Portfolio II. A similar project is pictured on page 66 of that book. They altered the directions slightly, giving a pattern to enlarge on page 107 of the same book.