Start with a rectangular sheet of paper
Place the paper flat with one long side facing you
Fold the bottom edge up about one-third of the way
Fold the top edge down to overlap the bottom fold
Unfold both folds
Fold the left and right sides inward toward the center
Overlap the side edges slightly
Tape or glue the side seams
Fold the bottom flap upward to close the pocket
Seal the flap with tape or glue
Insert the paper or card
Fold down the top flap to close the envelope