Start with squares, not triangles or octagons. What I did was mark the diagonal on the small square and place it in the corner of the larger square. DON'T sew on the line, sew parallel to the line, just a bit to the corner side. Do not trim, but finger press the triangle back. This will tell you immediately if it's in the right place. If it is, then you can trim the seam. If not, you can either take it out & try again or if you think you can live with it, cut the excess of the small square, but leave the large square in place and trim the small triangle to match.