Today I pieced batting for the second time. I just have too large a stack of batting pieces to open a new package of the increasingly expensive stuff! I'm just making this up as I go, but I lay it out on the quilt top - using as few pieces as possible. Today I had three pieces - last time just two. I butt the straightest edges up against each other and zigzag right over the join - using a fairly large zigzag. This one had two large pieces that I joined horizontally - making sure the join was not in the same place as the seam piecing the backing of the quilt. It lays very flat and seems secure. The third piece for this one was a long strip that went down the side. Not happy about it, but that's what I did. that long strip is about 6" wide.
I'm quilting it myself on my domestic machine. I've done about 1/2 the quilt and cannot tell where the batting seams are as I sew.
Any other advice? This is not a quilt I expect to see much daily use or to be washed frequently. If I can cut down on the collection of batting bits and pieces I have I'll be a happy camper.