Seams lining up starts with accurate cutting. If your pieces are out even 1/16 th, it does add up over several pieces and before you know it, your seam is out 1/4 inch. It helps to pin your intersections and if you need to ease in extra fullness, put the bigger square next to the feed dogs when sewing the pieces together. The feed dogs will use up a little extra fabric. Some people like to match up the seams and put a tiny drop of Elmer's washable school glue to hold the pieces together. Use a dry iron to temporarily fuse the pieces together with the glue.