The instructions are here:
All I can say is follow the instructions to the letter. Do not deviate from the instructions in any way. I had all sorts of problems, but they were all related to mistakes I'd made following the instructions. Eventually however, I did get the Samba service to start without any errors.
So I'm all set, right? No... Although everything appears to be running correctly locally, I cannot connect to the machine - as a domain controller - across the network, and I cannot connect via Fedora Management Console from another computer.
It really shouldn't be this difficult.