WickedWhims has fully implemented objective based exhibitionism skill that decides what you can do. Every level unlocks new possibilities as your sim gains more confidence. To start just change into more revealing outfit and show yourself to other sims. Because Sims 4 doesn't have anything that defines revealing of given outfit part the revealing outfits are swimwear and sleepwear (both don't include underwear). The more sims see you in revealing outfit, the more you gain to your skill and this rule applies to anything you do in the future. With every level you get a notification explaining new things you can do, so from that point everything should be clear.
Really cheap and easy way to force levels early is to use the mirror.