Start with a rectangular sheet of paper
Place the paper flat with the long side horizontal
Fold the bottom edge up about one-third of the way
Fold the top edge down so it overlaps the bottom fold slightly
Unfold both folds
Fold the left and right sides inward toward the center line
Overlap the side flaps slightly to form the envelope body
Fold the bottom flap up to close the bottom
Press all creases firmly
Insert the letter or note
Fold the top flap down to seal the envelope