When working with fabric that you are cutting at an angle, I always starch and iron fabric first. This keeps that bias edges from stretching, somewhat. With diamonds the seams will bypass each other when you put two together as someone else mentioned. To get them in the proper position, you let the seams lines form an X and put in a pin at the 1/4 inch seam line. Open up your pieces, checking the points and if needed re-pin the sew.