Step 1:
I glued four 20x20 centimeter papers together into one large Origami paper. Both sides have a different color.
Fold into three parts of the same size.

Step 6:
Prefold the edges to the prefolded line, see arrows.

Step 5:
Prefold over this line.

Step 2:
Fold over the upper layer.

Step 4:
Prefold into three parts of the same size.

Step 3:
Fold the red part backwards and turn the model.

Step 10:
Make the same folds on the other side, see arrow.

Step 8:
Prefold over this line, see arrow

Step 9:
Fold both edges towards each other, see arrows.

Step 7:
Use the crossing point of the two prefolded lines as guidance, see arrow.

Step 16:
Fold over these two lines.

Step 15:
Tape / glue both flaps together

Step 12:
Pull the corner to the left, see arrow.
Do the same on the other side.

Step 11:
Fold the edges straight upwards, use tape or glue

Step 14:
Prefold over this line, do the same on the other side.

Step 13:
Make the same folds on the other side, see arrow. Use the prefolded line of step 8 as guidance.

Step 18:
These two parts should be on the same height, see arrows.

Step 17:
Fold over these two lines, do the same on the other edge, see arrow.