It's easy to do the first block with all 6-inch strips. For that, you need to do partial seaming on the first strip. Here is a tutorial on how to do that:
http://quilting.about.com/od/piecing...tial_seams.htm
For the 12-inch block, since you need to do a mitered seam at the top anyway, I would just add mitered seams at the 2 corners (right and left points of the on-point square). The rest of the bottom would be basically strip-pieced, with a "set-in" corner on each side.
Clear as mud?