There are 2 different ways: the quad party storyline, or the dorm room storyline
Quad Party: Differs if you're a girl/fem or guy/masc character. For a girl character you need to get some sexual experience (specifically experience swallowing during oral so you can notice something is up with the milkshakes). For a guy you need to get a scene where one of the guys passing out milkshakes says something weird.
Then go in and out at the party till you get an option to investigate a noise.
Dorm Room: Take a shower in the morning to possibly trigger a random event with the BFF. A week or 2 later another event can trigger when going from the hallway to your dorm room late at night.
After completing EITHER of these (you don't have to do both!!) just keep visiting your BFF in the evening and talking about sex.