or you could cut one 9.5 inch square for the center block and cut four 5.0 inch squares for the corners. Someone check me on the 5 inch squares to double check I have that right. Place a smaller square facing right side down in the corner of the big square and stitch diagonally across the center of the smaller square. Trim off the excess from that corner and press your triangle out. I sew all 4 small squares on and press those "ears" out before I trim anything off. Basically what you have for your total block is a 4x4, or 16 patch block in other words. Break that down to each of the squares equalling 4.5 inches finished. So that makes your center square 9.0 by 9.0 inches finished.
Edit: yep, you want 5 inch squares for the corners. I checked and the small squares will meet at the seam line to form a perfect point 1/4 inch in from the edge which is what you want them to do. With a 5 inch square, stitching is much easier if you use a ruler and pencil in a diagonal line from corner to corner. Sew on that line.