In order to get the top and back to line up correctly, I hung them up and pinned them together along th top edge. Then both pieces were set on the long arm roller and pinned in place. I carefully set the batting down between the layers and repinned it with the batting one pin at a time. The backing was rolled onto its roller bar and stretched just enough to keep it firmly in place. I like to "float" my top.
Here's what it looks like on the long arm.