You'll need an envelope, decorative tape, and a scissors. I used 4 3/8" x 5 3/4" envelopes, but any size will work.
Seal the envelope and trim off one of the short sides to make an opening.
Wrap tape around both sides of the envelope. No need to cover the last inch, since it will form the bottom of the bag.
Fold about one inch of each side of the bag toward the middle; then fold the bottom upwards, creasing your folds well. You can vary the width of these flaps depending on the proportion of the bag you want. Flip the bag over and reverse the folds along the same lines for easier shaping in the next step.
Put one hand inside the bag, and with the other, push the bottom fold down. This will leave the bottom corners of the bag standing up. Press each corner into a triangle.
Fold both triangles down and secure them with more tape, covering the bottom of the bag.
Sources of fancy Japanese tape include Happy Tape, Cute Tape, and Ginko Papers. However, you might be sorry if you click these links because money will immediately fly out of your wallet. Be warned.