Here is a technique I learned but did not practice.
Lay two pieces of fabric that you want to have a curved seam face up and overlap them by about an inch or so. Using your rotary cutter free hand cut the curve in the overlapped fabric. Flip the fabric so the right sides are facing each other. You will have a very mismatched curves. Start stitching them together by lining the edged of the mismatched curves. You may need to work slowly in the beginning. Press well. This technique was explained in one of Rayna Gilman's books. That woman is a genius.