The book I have is sewing curved seams from straight pieces, don't remember the author off the top of my head but essentially you take two pieces of fabric at least 6 inches wide and lay them right sides up on top of one another. Then you take your rotary cutter and cut gently free hand curves across the middle lengthwise. Then you put the top piece of one side to the bottom. Then you take the two pieces of the two different fabrics and sew them together but you do not use pins. You use your hands to gently bring the edges together and sew, you may only take a couple of stitches before you have to stop and pivot the fabric to meet again at the edges. If you take some scraps and play you get the hang of it pretty quick.
The other option is a pen that just came out that you use after the seam is sewn and it helps the fabric bend to the curve for pressing. I will have to look at the name of it when I get home.