There are several youtube videos available for "how to sew a cylinder"
If you want the batting edges to be butted together - I would use the widest serpentine or multt-stitch zigzag you have on your machine to have a smooth joining.
Taping the edges together before sewing might make the job easier. I think there is some kind of fusible available for joining batting pieces. I have not used it, but others may have some advice about it.
Practicing on a small sample might give you an idea of how the batting will behave.