Start with a square sheet of paper, or trim a rectangular sheet into a square
Place the paper flat with one corner pointing toward you
Fold the bottom corner up to meet the top corner
Press the crease firmly, then unfold
Fold the left corner to the center point
Fold the right corner to the center point
Fold the bottom corner upward to cover the lower part of the folded sides
Fold the top corner down to form the envelope flap
Tuck the top flap into the pocket created by the side folds
Press all creases firmly to secure the envelope