I believe you need to have learned the Devil's Lust spell and maybe slave mode, although the girl does not need to have been offered to Lily (which I think is an oversight, but no biggie) and I think you need to have met with the strip club owner and talked to him about pimping your girls. After that, take a level 4 girl to the Airport Brothel Lobby and talk to her. If you have everything (and she's enabled in the events list for prostitution - for instance I'm not sure the baker is yet since she only just got her level 4), you should get a prompt to "use Devil's Lust to pimp soandso?" Choose yes, and if you want to watch choose peep.
AFTER that, go into the girl's status and click the $ icon. It should turn green, and she'll go into slave mode and begin whoring after school or work. You'll get income on day change. If you want to watch, enter the room she's in. Each girl has 3 scenes - one for each room. Up to three girls can be pimped at once. If you want to change girls, deactivate the $ on one of your current whores. I think that if you disable slave mode manually she'll stop whoring herself but the icon won't deactivate but I'm not super sure about that.