First off, this is from memory and I might have some of it off / wrong (especially since I haven't looked at the code that much).
If you mean Karryn laying down on the table, you need to get drunk (which is affected by endurance IIRC). So you need to drink when offered (set in waitress menu) and if a patron has a drink and kisses Karryn they can force her to drink too. Better odds if you don't fix her clothes, since they'll stop asking her to flash if her tits are already out. If you don;t need to be getting drinks for anyone, hang around whichever table has the most patrons with liquor.
Then, every turn there's a chance for it to fire, with a higher chance the more drunk she is. IIRC you lose drunk status over time as well (again based on endurance), so you really need to keep up the drinking and having only beer on the tables makes it harder, since you get drunk faster on hard liquor.
CCmod also has some changes that increase the odds of this happening, you can give the wrong drink which pisses off the patron and they can force you to drink and then a separate chance for them to claim they got the wrong drink even when it was right.