Step 1:
I taped three standard printer-/copy papers into one long paper of about 90x20 centimeters.
Prefold in half two times

Step 6:
Use these two spots as guidance

Step 5:
Unfold.

Step 2:
Prefold, both lines are about 2 centimeters off the centerline

Step 4:
Fold the edges to the prefolded lines

Step 3:
Prefold, both lines are about 4 centimeters off the centerline.

Step 10:
Repeat steps 7 to 9 on the other side

Step 8:
Press flat

Step 9:
Fold over this line

Step 7:
Fold over this line

Step 16:
Prefold these lines into the right direction

Step 15:
Fold in half one more time.
Then unfold the folds of step 13 to 15.

Step 12:
Prefold into 3 parts of the same size

Step 11:
Prefold, both lines are just a little off the edges.

Step 14:
Fold in half.

Step 13:
Fold both parts in half

Step 21:
Glue/tape these two spots to the model

Step 19:
Fold/press these lines against each other. Do the same on the other side

Step 20:
Press these two spots aside and press the purple lines further against each other, also on the other side.

Step 18:
Fold/press the lines between the arrows against each other. Do the same on the other side.

Step 17:
Fold the edges straight upwards

Step 22:
Press/fold these lines against each other. Do the same on the other side.

Step 25:
Tape/glue these lines together, do the same on the other side

Step 26:
Press/fold these two spots downwards (make the same folds on the other side)

Step 24:
Press these two spots aside and press the purple lines further against each other, also on the other side.

Step 23:
Press/fold these lines against each other. Do the same on the other side.

Step 27:
Tape/glue these lines together. Do the same on the other side.