Gather a sheet of paper
Place the paper flat in portrait orientation
Fold the bottom edge up to create a small flap
Fold the top edge down so it overlaps the bottom flap
Fold the left side inward toward the center
Fold the right side inward toward the center, overlapping the left side slightly
Unfold the side folds
Fold the bottom flap up again
Fold the left and right sides inward again
Insert glue or tape along the side edges of the bottom flap
Press the side folds down to seal the envelope
Let the adhesive dry if needed
Insert the letter or card
Fold the top flap down to close the envelope
Seal the top flap with glue, tape, or a sticker