Step 1:
Take a large piece of wrapping paper.
Prefold.

Step 6:
Fold to the centerline one more time and unfold the model.

Step 5:
Fold the edges to the centerline.

Step 2:
Cut along the pink edge.
Make 2 large squares this way.

Step 4:
Fold the edges to the centerline.

Step 3:
Prefold in half over both lines.

Step 10:
Both mountain-folds are prefolded

Step 8:
Fold over these two lines, the mountain-fold is already prefolded.

Step 9:
This mountain-fold is also prefolded

Step 7:
Repeat steps 4 to 6 in this direction.

Step 16:
done.....

Step 15:
Use glue to connect the edges

Step 12:
Both mountain-folds are already prefolded

Step 11:
Repeat step 10 over the whole model, see arrow

Step 14:
Fold both edges backwards

Step 13:
Repeat step 12 over the whole model, see arrow.

Step 18:
Prefold and cut in half.
Then fold both rectangles in half over the length a few times.

Step 17:
Prefold and cut the other large square in half

Step 19:
Glue/tape both strips together and fold the ends in half.
Glue/tape both ends to the inside of the bag.