I piece batting scraps after overlapping and cutting through the 2 layers with a rotary cutter. Then I lift away the trimmed slivers and while the 2 pieces to be joined are still on the mat, I use a blue chalk wheel to mark registration marks across the join at about 3 inch intervals. That way when I am joining the 2 edges at the sewing machine, I do not have stretching as I sew the lengths together. I use the widest and longest 3 stitch zigzag and very thin ivory serger thread so no bulk is added. I just butt the 2 edges together and sew. Then I add another piece and pretty soon it is big enough to be useful. Good luck!