17.
Apply four valley-folds of the step #16.
Prepare to apply three mountain-folds and one valley-fold.
As for three mountain-folds, you need to insert the flap into a pocket after applying the mountain-fold.
18.
Apply three valley-folds and one mountain-fold of the step #17.
Be sure to insert each three flaps into a pocket after applying a mountain-fold.
Prepare to rotate the model by 45 degrees in the clockwise direction.