I stitch from the outside in until it comes to putting together the two 'halves'. Then I carefully mark the center point of each half and start stitching to the outside. Then go back and do the same thing for the other side of the "half". I found that my center meets perfectly when I break it in two.
BTW, I wasn't doing OBW's but 30 - 6" compasses, but the assembly should be the same.
Diana