The back is the same size squares, but solid fabric. So after pinning both sides roughly matching up the seams, I quilted in a back and forth pattern down all the seams. That's all I could manage with the bulk of the quilt rolled up and shoved inside the machine arm. There's really not much room to maneuver.
Then when it was all secured like that, I went back and filled in the blocks with random stitching, because I was able to wrestle the fabric around, not having to worry about accidentally scooting the backing.
"Just spraying thread around," DH called it :-)