I would lay both piece of fabric right side up with the sky above the trees. Overlap the edges by the amount that you think will accommodate the ups and downs of your hills -- hills on top. Then either use a template or free-hand your hills and valley cutting through both pieces of fabric -- you want gentle curves to make it easier to sew the two pieces together. Put the pieces right sides together and sew carefully -- you may have to stretch the curves a bit to make them fit. I then trim the curves slightly with pinking sheers make them lie flat.