You need to put registration marks on the edges of each piece. Lay the two curves you want to sew together beside each other, back side up, and mark a line from one to the other across the cut in several places (kind of like marking notches when cutting sleeve and shoulder pieces of a blouse). When you pin the two curved pieces together to sew them, match up the marked lines. Pinning fairly close together helps, too. Curves need to be eased, even gentle ones.