Usually if you have the sheet metal license you can go into Application>Sheet metal>Forming flattening, and tools flat pattern. This will create curves of a flat pattern.
It is quite good for most simple applications and somewhat capable of keeping up to date with changes to the part design associatively.
There are advanced techniques associated with unfolding shapes that are other then flat with simple flanges. This usually means pressed sheet metal parts.
What John has alluded to is that with an unfolded solid you had extract the curves etc as required. If you are already at that stage then your next step should be straightforward.
The flat pattern technique that I described has the advantage that you could equally derive the flat pattern for parts not created with sheet metal flange features.
Best Regards
Hudson