No, really. That's it. I took the waist band of the petticoat and pulled it all the way up until it touches the bottom of my bra band. This pulls the fluffy part upward, creating a base for the skirt to sit on, creating the lovely bell shape we all want. Here's a photo of what that looks like:
Here's a before shot:
And and after shot:
See the difference? I am using the Babyonline Rockabilly Tulle Petticoat, and, full disclosure, I also sewed a couple more runs of tulle to the underside to give even more lift, mostly because I could not find a petticoat that was just the right fullness. I found that most that were advertised as fluffy were too fluffy and the rest tended to go flat. So I took this inexpensive one and added to it, and voila! POOFINESS.