But you can sew flat curves !
Overlay your two fabrics, wrong sides up.
(Bad photo, my grey one is under, you can see it on the right…).
Draw your curve.
(As you are on the wrong side, as I did, you can modify your line).
Take off the right piece of the first fabric and the left piece of the second one.
You should obtain two shapes which coincide.
Turn your fabrics right sides facing.
Mark some points with pins.
Forget you are sewing curves !
You have to sew straight.
Moving gently your fabrics (don’t stretch them !), put the edges together and sew slowly.
To avoid folds, don’t try to sew too quickly !
Warning : even if you sew carefully, at the end your fabrics could be crooked.
Provide a bigger size at the start.