All great advice, start in the center, use a walking foot, iron the puckers out. Somebody else was told to do the borders first in a quilt class, that doesn't make sense to me, because if you are off one tiny little bit, you will get a pucker in that last corner. I always work from the middle. I'm a pinner, and I use a lot of pins. My backing is pretty flat because I clamp it to my cutting table before I add the batting and top. I pin from the center. And I use an Accufeed foot (same as a walking foot). Once I did all those steps, I very rarely have a pucker, and if I do it might be on the very last corner of the quilt as I'm doing the last stabilizing before adding the border. I can live with that.