When you sew your sets of three together the point made by the center one should be pretty close to 1/4 from the edge of the fabric of the two outside ones. The cut point on the center diamond will be a lot longer. Then when you sew them together it should come out all right. Take it apart and try again. With diamonds if you sewed the 1/4 from the point and not the side it will come out wonky. I found, when doing these for the first time, that it helped if I used a ruler and a washout marker and marked my 1/4" with a V or X at each point.