A few notes from the designer:
When you print the file, be sure to print the page with no scaling, or the cut and fold lines will be all wopsy. Make a vertical cut down the center at 4.25". Then make a fold 2" from the bottom and another 6.25" from the bottom--that should leave you with two flaps and a 4.25" square in the middle. Fold the two white flaps over one another and seal with tape or a dot of glue or a sticker. As always, print as many as you like and give them away freely, just don't sell or redistribute them electronically, s'il vous plaît.
Link to Cottage Industrialist's blog post and PDF design here.