I lay out my batting pieces on the wrong side of my backing material and spray it with basting spray. Top it with my quilt top and smooth it out. Turn it over, lift backing and spray batting pieces and lay backing on top. Smooth it out. Works well and I don't have to fool with piecing the batting pieces.

I use a glue stick and try to just overlap very slightly.

This works well for me!

If the zigzag seam is bunching up/pulling too tight, you might have to reduce your thread tension slightly.

Great! Here's to no more Frankenstein Scars! :)

I do this all the time. The real secret is in having straight edged. Otherwise they just don't fit well.

The only time I tried using the machine, I got the stretched out Frankenstein scar. Now I do mine by hand. Big overcasted stitches don't take too long.

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!

I also overlap and do a gently curved cut. Remove the excess and push the pieces together. I actually use the open toe foot with a wide zig-zag and adjust as needed. Maybe you need to decrease the foot tension on your machine.

I found this video quite helpful for piecing batting.... I hope it helps!

https://www.youtube.com/watch?v=IGNBpBJ-8HE