Choose a sheet of paper
Place the paper flat with one corner facing you
Fold the bottom corner up to the top corner to make a triangle
Press the fold firmly
Fold the left and right corners toward the center so their tips overlap slightly
Fold the bottom flap upward to cover the lower part of the side flaps
Tuck the bottom flap into the side flaps if needed
Press all folds firmly
Seal the envelope with tape, glue, or a sticker if desired