Start with a square piece of paper
Place the paper flat with one corner pointing up
Fold the bottom corner up to the top corner
Press the crease firmly
Fold the left corner toward the center, leaving a small gap at the edge
Fold the right corner toward the center, overlapping slightly if needed
Fold the top flap down over the two side folds
Tuck the top flap into the pocket formed by the side folds
Press all creases firmly to secure the envelope