The link from auntpiggylpn is very good except I'd rather take out 3-4 stitches than stretch the fabric. I just used this technique for the night and Day quilt when I joined four blocks to make a 12" block. It worked beautifully except where I altered joining the seams on one row of the seven. I nested the seams and stitched but in the wrong direction so that when I joined the rows, I could not nest the seams because they were going in the same direction. I had to rip out a little to fix this. The seams are all now beautiful and I think it all has to do with the nesting of the seams. Hope you understand what I'm saying.