Are the blocks on the back going to be the same size as those on the front? If so, I would sandwich the three layers together and then fold down the top to the half way seam and alighn with the matching seam on the back piece then secure with pins. then fold to the next cross seam line and do the same etc etc etc. And then do the same for the other side.
If the blocks are not the same you can still do the same by marking these alighnment points on the back one.