I have made my first petticoat. It isn't super great, but it's fluffy and fits and I'm digging it. I didn't use a pattern, but based it off of this image:
The petticoat has a placket onto which I'm going to put hook and eye closures (it's only pinned on in this picture), as well as a drawstring since it has a tendency to pull away from my back due to the bustle. I also added another button to my bustle since I was having an issue with it shifting once the petticoat was on.
The second fitting of Castiel's corset went beautifully, and I'm ready to start working on it. I still don't have a busk, but I can do everything else up to that point.