The size of the pages from this book after trimming is 6 1/2 inches by 10 inches. Any size rectangular page will work with this series of folds. The size of the finished envelope from this page is 3 1/4 inches by 5 inches. The larger the paper, the larger the finished envelope, of course.
The first fold: Fold the right edge of the paper at an angle as shown with the top of the page about three inches in and the bottom about one inch.
Fold two: Fold the left edge of the paper over and line up the two edges as shown.
Fold three: Turn up the bottom of the paper about 1 1/2 inches and make sure that the right and left edges created by the first two folds are flush on each side.
Fold four: Fold down the top and overlap the edge of the bottom flap by at least a 1/4 to 1/2 inch, again making sure that the right and left folded edges are flush.
Now tuck the bottom (or the top) into the pocket on the opposite side which was created by the first folds. This will lock the envelope together. I usually put a sticker or an address label over the middle of the envelope where the top and bottom meet to seal it shut. A small card can be enclosed before finally sealing the envelope. If the paper is blank on one side, a letter can be written before the folds are made. The outside can be decorated before or after it is folded.