[10] Valley fold as shown starting at point X. The fold line should be parallel to side edge YZ. This will create a rectangular flap.
[11] Fold the top corner of the flap down as shown.
[12] Raise the flap slightly.
[13] Open the top corner of the flap by pulling on the outer layer of point A, then flatten the corner down to form a triangular flap. Figure 13a shows you how to do this.
