You can shrinkhammer the flange if you want the flange go inwards from the curvature or strech hammer the flange if you want it to go outward from the curvature.
Here's an example.
Even though these panels are flat the flanges are curved. The same issue with them. On a outer curve the flange would wrinkle if you just hammer it down but with a bit of practice you learn the technique to shrinkhammer the wrinkles before they wrinkle...
And on the inner curves you simply have to strech the metal to be able to form the flanges.