You can also strip piece the sashing pieces with cornerstones - your measuring and sewing has to be pretty precise.
Example ... a six inch square with a 1" sash, sew the side sashes on all the squares so you have a long row. Then, make a strip set with a 1.5" strip (cornerstone), 6.5" strip (sash), 1.5" strip (cornerstone). Then, crosscut the strips into 1.5" strips. You'll end up with the cornerstone/sash/cornerstone to fit on your sashed squares, without dealing with fiddly little squares.