Diane, Believe it or not, never discovered a good way to do it, but a quilting buddy suggested making the 10" & adding an extra row of background fabric to bring it to the needed 12". Mine was for a sampler wall hanging, so it worked well & looked fine. Elenore Burns has a pattern for 12" FD in her Quilt Blocks on American Barns, that I bought after I sewed my block. Hope this helps.