Just making it square won't do it. It has to be the "correct size" square. The hour glass square must be exactly the same size as the plain square, with the seams going corner to corner. When you trim, be sure the diagonal line on the ruler is on the seam line, both ways. Then when you sew the two together, you will get what Peggi showed. Now the diagonal seams don't go to the corner any more. This is correct. That is your seam allowance for the next seam.
Hope you can understand this and hope it helps.