Bow and arrow tutorials, this way!
For a ballgown, there are two ways of doing it (and a third, if you do a hybrid), which would be either with petticoats or with a hoop skirt.
Hoop skirts you can buy online for between 20~30USD or so. I’ve gotten them off of eBay before. These are skirts with horizontal ribs of hoop steel sewn in. The hoop steel forms a ‘cage’ of multiple complete circles that get larger toward the bottom of the skirt, forcing the skirt out into a bell shape. Hoop skirts can get you very large skirts with minimal weight and added temperature (the airflow underneath helps keep you cool), but require at least one layer on top to prevent the hoops from showing through and can be difficult to maneuver in. These are good for very large skirts that are somewhat rigid.
Petticoats are made out of many layers of tightly gathered fabric. Often, the fabric is stiff for added poof (typically tulle or a stiff netting, sometimes organza), but chiffon “softie” petticoats are popular in both square dancing and lolita fashion, as they never lose their stiffness/poof, and sometimes cotton or linen petticoats are used in historical costuming. These are good if you want a very particular shape (since you can customize the shape by using different cuts of fabric) or if you want less poof or more movement. These can take a lot of time and skill to make, though. Often, horsehair braid or fishing line is sewn into the hem of a petticoat to give it more body, and sometimes a smaller hoop skirt is worn with larger petticoats over it (as opposed to a smaller petticoat worn with a hoop simply to prevent the bones from showing).
Which one you will use is up to both personal preference and what the costume looks like. For example, this could be either (or both), this would be a hoop, this would be petticoats, and this would likely be both. Keep in mind that with petticoats, in order to get more poof, you’ll need a /lot/ of fabric, so that’s where hoops and other understructures come in handy.
I hope that helps! :]
(By the way, you get the honor of being my last ask on this blog. It was great, everyone! :])