Here are instructions on how to make flat centers on the back of a quilt block that has an integrated center:
At the middle of the long seam on the back of the quilt block you will see that there a couple of threads that need to be clipped. Clip only those couple of threads into the seam allowance. Press the long seam in two different directions. This pressing creates a miniature 4-Patch design which will eliminate bulk.
This process is the same for the back of a Pinwheel quilt block. This method creates a cute miniature pinwheel!