Best way to do it as far as I'm aware is to get the arousal bar up really high without any sex or masturbation, though do remember that you get diminishing returns for it
Mmmm.... It worked.
[7] Sell arousal-stained pink dragon-scale print thong (Stained)
I'm not sure though what really caused it. I did try to stay aroused a lot (two weeks or so?) but in the end directly before I discovered that the panties were stained some guy fingered me to orgasm on the bus. So yeah, not really sure.
But I also bought cum-stained panties I found in the thrift shop in town and that worked as well. My fans are perverts :)