I realize this might be a dumb question. But I ordered a few dresses (pics to come soon) and I realized I don't know what kind of underthings to wear. Like, do I need a slip? Spanx? Regular undies?

My goal is general smoothing-out/avoiding VPL/avoiding static. I have some generic Spanx-style shorts thingies but I feel like they are not tight enough to do anything and they're also hecka ugly. If anything they make my thighs look fatter because they only compress the upper part of my thighs. If you do wear Spanx shorts, do you wear undies under them? I feel like a kind of fitted slip might be my best choice but so people still wear slips? I literally can't dress myself lol