This is a plain and simple fold. It lends itself well to letter head paper with a contrasting border which becomes the focal point of the folding.
It looks like a standard oblong envelope. It it were not for the diagonal edges, that could catch on sorting machinery, you could send this letter fold through the standard mail system.
If you really want to do this then I suggest that you pack it inside a standard envelope.
Alternatively you could make a transparent one with Gorilla Glue and clear Grafix Dura-Lar, Dura-Lar is an acetate alternative that holds a crease, unlike many other plastic type materials. Unlike acetate, it does not crack, tear or chip. It is wonderful stuff for origami enthusiasts. If you live in the United States, you can buy small amounts from Dick Blick stores or larger amounts directly from the Grafix web site. I imagine there are similar materials available in other countries.
Pre-crease you model before folding. Then follow the directions for collapsing it. If you start where indicated you will have a much easier time tucking the last corner under the adjacent folds.
There are few more tricks which improve accuracy and ease of folding. These are shared in the full set of diagrammed instructions which is now ready. If you are a seasoned folder you will most likely discover them for yourself as you try out the model.
