This requires some moral fortitude to successfully pull off. If you don't REALLY want to do it, you straight up won't. This is an act of breaking up or ensuring that the girl/guy you do this to NEVER calls you again.
1. You have to have sex with them first, other wise it's just not as effective.
2. After sex when they're asleep you've gotta take a crap somewhere other than the toilet.
3. Here's where the moral fortitude comes in, you'll need a ladder first and some thing to pick up your crap with.
4. Pick up your excrement
and place it on top of the fan blades, spread it out, the smaller the pieces the better the effect.
5. Once all of your crap is safely on the fan blades get dressed and gather up all of your belongings in order to make a quick escape (did I mention this can only be done at their house, NOT your's.)
6. Start flicking the lights on and off and screaming until your unfortunate victim wakes up, once they start moving and moaning turn the fan to max and sprint the fuck out of there.
7. Never agree to meet them in person ever again as you'll be promptly castrated or killed.
8. If done right and fast enough the majority of the walls and floor near the corners will have little flecks of crap EVERYWHERE. If done wrong it might fall off the fan before you start, or it will create a few lumps of crap that are relatively easy to clean up.
TIP: if you feel like you may have taken to much time or your stool
was a little dry it's okay to wet the fan blades before you turn it on.
Kenny: you know that fat bitch linda who's been sweating me for the past month?
Me: Oh No, you didn't have sex with her did you?
Kenny: Yeah, but it's okay cuz I pulled a Dirty Thunderstorm on her after we finished. I Made It Rain!!!
Me: You sick Bastard! (Laugh)