I generally overlap the two slightly on one edge (slightly = 6" max). then I'll use my rotary cutter to make a wavy line (I avoid straight lines as they will be more likely to be visible!). Make sure that when you cut, you are cutting through two pieces - this is necessary to butt the two pieces together exactly. Just a nice gentle long wave. then I'll pull off the excess on each side - then butt the two pieces together. It should be very easy to butt because the waves will match.
After that, I hand sew the two edges together with a zig-zag going through the middle of the batt. I use as fine a weight of thread as possible. Make sure when you pull your zig zag stitches together you don't distort the edges of the batting.