Step 1:
This paper is about 20x40 centimeters (Two squares). One side is yellow and the other is green.
Prefold over these two lines.

Step 6:
Prefold the left edge to the prefolded line of step 1

Step 5:
Prefold all (prefolded) lines into the same direction.

Step 2:
Fold both edges to the prefolded centerline.

Step 4:
Fold both edges to the centerline one more time. Unfold the model afterwards.

Step 3:
Fold both edges to the centerline.

Step 10:
Make pleats, the two brown mountain-folds are prefolded lines. The two white valley-folds lay very close to the mountain-folds, about 1/4 prefolded square distance.

Step 8:
Fold the part between these two lines in half 4 times (see arrow).

Step 9:
Prefold these (prefolded) lines into the same direction.

Step 7:
Fold the left edge to this prefolded line 4 times. Unfold afterwards.

Step 16:
Fold these two mountain-fold lines to the right, see arrows.

Step 15:
Fold over these two lines.

Step 12:
Repeat step 10 in this direction

Step 11:
Repeat step 10 over the length of the left part, see arrow.

Step 14:
Fold over this line

Step 13:
Repeat step 12 five more times, see arrow

Step 21:
Fold about four more spots on the upper and four on the lower edge downwards.

Step 19:
Glue the left and right edge together, see arrow.

Step 20:
Fold these two spots on the upper and lower edge downwards.

Step 18:
Fold the flap on the left and the flap on the right backwards over the brown lines. And fold the two upper edges backwards, see arrows.

Step 17:
Fold these two mountain-fold lines to the left, see arrows.