NOTE: I have just posted diagrams for Nick Robinson's Memphis Belle at the link below:
Make a mark 2" from each side at the top of the paper, then make two more marks 1/2" above the bottom edge of the paper on each side.
Make valley folds as shown.
Make a valley fold approximately 1/2 inch from the top. Make sure it is a crisp crease. Make eight such folds, making sure they are tight and clean.