For batting, you would cut both edges straight, then just butt them together, don't overlap. Use a wide zig zag or similar stitch to sew them together. As far as piecing the backing, just try and avoid having a backing seam and a seam on the top right on top of each other. The backing seam will cross some of the seams on the top, but just try and minimize this. If you do a search on set in seams on this board, lots of tips will come up.