I sew my squares top to bottom and sew back to the top. when I am ready to join I sew left to right. I also always sew a straight seam line across a corner of my first block so that I know that is the beginning of the first row. One time I did not and had to take it apart. I had sewn the bottom to the top of the next row. When I am ready to make the sandwich I take the line of stitching out. Have not had trouble with the right ends together since.