The 6 photos in the web link above show how to piece batting. The photos [and their titles] pretty much explain the whole thing, but here is a little additional info:
Piecing is done in a wavy line to avoid feeling or seeing a ridge or line in the finished quilt.
The wavy line should be large gradual curves.
The stitching I've done in black so it would show in the photos, naturally you would use thread to match the batting.
A single thread in your needle is all that's needed.
Neatness doesn't count when you stitch this...I started at the top and used big sloppy overcast stitches. Then I started going back up with the stitching to show the completed stitching. You would sew one direction for the whole pieced seam, then head back the opposite direction to complete the seam.
This will be the quickest hand stitching you've ever done :)