What I would do would be to find the exact place on each piece that will match. I'd put a pin in the two pieces at that place and then I'd put the right sides of the fabric together placing the two pins one on top of the other. I'd then pin the two pieces together and open them up to see if they actually match. If not I'd try again. Once I had the two pieces matching perfectly, I'd pin them together all the way down the edge and then cut the seam allowance at whatever distance from the pins I wanted. Then I'd stitch along the line of the pins. I hope I've made this clear.....it's hard
sometimes to visualize written directions. Personally I like to see pictures of what is being described, and I would include them here if I knew how! :-)