The problem with getting a smooth inner circle appliqués is the seam allowances underneath on the white diamonds. I think I would prepare my center circles (edges turned under with whatever method) position it over the white diamonds while on my table. I would then carefully carefully pin it in place at the 12,9,6,3 clock position and then lift up the turned edge between the pins and place a tiny snip of fusible on the diamond edge and iron down. Be careful that the fusible will be covered with the circle. Once it is all fused in place, I would appliqué in place.