You would cut your layer cake into 2 1/2 inch strips. You should get 4 of them. You would sew the 4 strips together (just like you would do if it was a full length jelly roll strip). Your block would end up being 8 1/2 x 10. Then you would trim each set of 4 strips down to an 8 1/2 inch square (just like you do with the jelly roll). I guess I'm confused but I don't see the difference.