I think you may be making it harder for yourself (we all tend to do that at times). You are simply adding a strip to a charm (5" square to a 2.5" x 5" strip). Then you are adding a 2.5" square (cornerstone) to a 5" x 2.5" strip and adding that the other piece. Take a look at just one block in the quilt and it will be easier to see.