After sewing two diamond segments together and press the use a ruler and make sure that the square at the points, if it doesn't readjust your seams and check again. Something people will want to trim it up, but then the diamond points will not be the same size. Same goes when sewing the two pairs together, put a straight edge all the center part and make sure it is straight. When all sections are sewn together you would want to press the seams to all go in one direction, open the center points and take out a few stitches and you can get the center seam allowance to swirl and it should lay flat when pressed with the swirl.