Step 1:
The folding paper is about 60 x 17 centimeters. I made it by glueing 2 different colors of wrapping paper together.
Prefold in half and fold both edges to the centerline.

Step 6:
Fold both corners inwards by making an inside reverse fold over the purple lines. The darker part of the two diagonal lines lays below the upper paper layer.

Step 5:
Fold over these two lines, use the prefolded line of step 3 as guidance.

Step 2:
Fold to the centerline and unfold the model.

Step 4:
Fold over these two lines, use the prefolded lines as guidance

Step 3:
Prefold over this line, use the prefolded lines as guidance, see arrow.

Step 10:
Fold this edge inwards.

Step 8:
Fold over these two lines.

Step 9:
Fold along both edges.

Step 7:
Mountain-fold over these four lines.

Step 15:
Press / fold the upper part of the bag flat and fold the flap downwards, see arrows.

Step 12:
Prefold over the purple line. Unfold the fold of step 11 and pull the flap upwards.

Step 11:
Fold the edge under the flap on the left, see arrow.

Step 14:
Fold these two inner edges downwards

Step 13:
Fold the left and right part straight upwards, use the prefolded lines as guidance. Then tape or glue the edges together.