I spray baste on the floor. I line the three layers up and then fold the top two back and start spraying from the middle going up to the top for each layer and then from the middle going down to the bottom for each layer, carefully smoothing as I go. My excess backing on the edges gets folded over onto the top and pinned to keep the edges firmly together when pushing/pulling through the machine.
I then quilt any continuous seam lines of large sections in the ditch from the centre out to stabilise the whole quilt and then I can quilt each section.
I have used this method for any size from table topper to full quilt.