Look at my diagram and fold along the lines. Red lines are mountain folds(away from you), blue lines are valley folds(towards you). All green rectangles are the same size.
A rectangle will work just as well as a square, just remember that the green rectangle in the middle is always the size of your envelope.
Completed folds. I accidentally made an extra crease down the middle-just ignore it.
(This picture is turned around. )
Fold the bottom part up along the crease.
Bring the sides together and put the left side into the right pocket.
Fold the top over along the crease.
Tuck the corner that's sticking out into the pocket.