This kind of thing is why we all learn to fudge a bit. *Ahem* Not s'posed to admit that one. All of the suggestions above are great. The only thing not mentioned specifically above, though hinted at is that the sewing machine itself can contribute to the problem. The feed dogs pull the bottom layer ever so slightly which causes the top layer to slip down a little smidgeon. I always use an anchor piece to begin and am strict about making sure the top edges meet and pin until a couple of stitches are in. Before getting to the bottom of the squares (somewhere around 1/2 way, make sure they also are aligned evenly and hang on to them for dear life (or pin).