That's very easy. There are alot of notes to leave. If he/she is a person that doesn't mind his business just buy a book and fill it with stuffs like.
I really need to join that cult
Am going to impress them today
Yes finally in.
Cult initiation today. Yes!!
Tomorrow is my first meeting with the other members.
I can't believe James used his roommate as an offering.
I wonder who I will use.
When it be my turn to bring an offering.
OMG I'm to bring next week.
Something like that. That's a bit scary .
Or you could use something that disgusts him/her. Anything that he/she hates big time, that will work fine.