cut the sashing 9 1/4 by the width that you want and the cornerstones the width that you cut the sashing by the same width. Hope you can understand that. Let me do an example.
Say you want the sashing to be 2" finished. You would cut the sashing 2 1/2 x 9 1/4 and the cornerstones would be 2 1/2 x 2 1/2.