Welcome to the board from Southern California!!!!
I absolutely hate piecing batting and will try my hardest to not have to do that.
That said, there are a couple of ways to do it. One way I learned was to overlap the edges of your batting about 3 inches and then using your rotary cutter cut a wavy cut. Then you can either hand sew back and forth to sew them together or use your machine and a large zig zag stitch. Now this is for cotton batting. I am not sure how to do a polyester batting beause I never use it.