For those of you who don't wear costumes as a matter of course, a petticoat is a garment you wear under your skirt to make it stand out in a bell shape. Think square dancers, poodle skirts and 80s wedding gowns. They come in two varieties; hooped and soft. We are not concerned with hoop skirts, here.
Modern long petticoats usually consist of two layers of polyester, with two or three flounces of net attached to the inner layer. The inner layer is usually A-line, and the outer layer is gathered and flounced. The inner layer keeps all that bristly net away from your skin (and provides a modicum of modesty in a stiff breeze), while the outer one keeps all that bristly net away from your skirt. Still, the stuff that actually does the job is the net.
These things need to be washed occasionally--definitely, if they are being worn on stage. They get sweaty around the waist and grubby around the bottom. In this day and age, most of us don't know how to do this. Enlightenment is about to ensue.
You can wash them in the washing machine. No, really...you can. Cold water, delicate setting, not too much detergent. Wash one at a time, with as much water as your machine will hold. Now comes the fun part. When you pull it out of the washer, secure it on a hanger, either with clothes pins, clips, or, if the petticoat has a drawstring, just draw it up tightly around the neck of the hanger, and hang it from a secure place. Here's the thing. The net will have got all squished up during the spin cycle, and it will not straighten out by itself. You have to pull the wrinkles out by hand, while it is wet. If you wait till it's dry, it will stay all bunched up until the next time you wash it.
Do not (I repeat), DO NOT put it in the dryer. If you do, you might as well throw it away, as it will be completely ruined.
If you pull the wrinkles out of the net while it is still wet, your petticoat will serve you for years to come.