3)  Unfold, then make another diagonal fold so
that the other corner meets the centerline at
the same point the first corner did in step 2.
The two diagonal folds should cross each
other at the centerline.
4)  Fold from the intersection of the diagonals
and centerline to the corner.
Images, Text, and 3-D Forms Depicted 2001 PaperAirplanes.net