well, if the sashing is going onto the sides of your blocks, it should fit. match center and ends. pin if necessary and sew. If you are doing long sashings cut the size of a row of blocks, you actually do the same. match centers, ends and pin, pin, pin. then sew. perhaps starching your fabric might help and don't pull it as you go. this should help you prevent it from getting wonky. Good luck.