Bobbinchick - I made one of these several years ago during a class. We used the iron on stabalizer with the grid markings on it and used rectangle fabrics - it was soo much fun and alot easier to work with rectangles instead of diamonds. But one of the tricks they taught us to get the center to lay flat is that once you have it pieced into the 4 sections (each with two diamonds) you have to "square" the 4 points that are going to meet in the center - then when you have two halves, make sure your edges that will meet at the center are perfectly straught. My center turned out perfect, not a bulge in sight.