Start with a rectangular sheet of paper
Place the paper flat with one side facing up
Fold the bottom edge up about one-third of the way
Fold the top edge down to overlap the first fold slightly
Press the folds flat
Fold the left side inward toward the center
Fold the right side inward toward the center so the edges meet or overlap
Tuck one side flap into the other if possible
Fold down the top flap to close the envelope
Press all edges firmly to secure the shape