you must press your row, then measure it for the horizontal sashing strip.
then lay it on the row matching the ends, and pin. then match the center and pin. it necessary (if long row) pin in between also. pins help and don't use an extremely tight stitch.
also use a foot that allows the fabric to move easily under the foot.
this one shows corner stones-
http://www.synthcom.com/~val/Quilts/Tutorials/AddingSashing.html