For the milkshake route.
1. Go to the Friday parties and increase your party reputation. Assuming you've passed most to every party skill check. Around the 4th to 6th time you go to the party. Someone will jokingly talk to you about the secret of the milkshakes. Or you can drink the milkshakes depending on a certain swallowed threshold (5+). You'll notice something special, and oddly familiar about the shake. Which will give you the quest to investigate it.
2. If investigated you will find out what the secret ingredient is. To which you can then tell your BFF. Then you both of you will go back to investigate. Which will end up with you caught. By a rich kid that will give you money to stay quiet.
4. You get the option to snitch and make a lot of enemies, or stay quiet. If you choose to stay quiet you and your BFF will have to add the special ingredient yourselves while being recorded. Which will go one of two ways.
5. Either you'll stand next to each other and handle your business making two milkshake. Or one of you will have to help the other handle their business, and then drink the milkshake on camera.
6. Afterwords you have to go to your BFFs room in the evening. Eventually you'll get the watch corn prompt. Which will end with you sleeping in their bed. You'll wake up feeling really hot and bothered. So will your BFF....
7. Afterwords you'll leave the room with your BFF following to talk about your relationship. Pick the relationship you want with them.
I suggest you go this route if you want/need money. Because if you agree to stay silent you'll get $400. You can still do this quest even if you're already in a relationship with your BFF. It's free money or you could go the snitch route. If you're a masochist who wants to be bullied, extorted, and/or forced into doing favors.