I've only ever completed the bff storyline using the shower option so let me explain. From what I've seen the timeline of events go like this.
1) On the first day of school talk about sex with then to open up the storyline
2)The party invite comes second week and the event in which you get your clothes stolen happens a week later which is the third week of the first month of school. After this on the call for a favor route the BFF will eventually ask you for help in completing a class assignment which you opens up that part of the storyline.
3) Provided you haven't gotten together with any NPC the event for help on the assignment should trigger a week after ONLY when you're approaching your dorm room late at night. This must happen specifically so at any time of that week walk up and down and roam around your dorm at late hours and the BFF should ask for help which you obviously accept.
4) After helping them out with the assignment fall asleep in their bed and don't leave their room afterwards and you should get two choices one of which will allow to start feeling new things for the BFF which moves things forward on that path.
And that's pretty much it. Every time after this visit their dorm every day of the week and talk about sex every time the prompt appears till yet another event fires off where you fall asleep in the BFF's room with them on their bed and depending on what you do both of you end up smashing. From there the BFF takes you out into the hall and asks you to define the relationship you want to have with them which varies from an exclusive relationship,an open relationship,a poly relationship or less romantically implied ones like friends with benefits. Your choice and the storyline ends. This is specifically for the shower/asking for a favor route as it's the route that is most convenient for me every time I do their storyline.
Also make sure your disinhibition is raised to at least level two to get that scene of where you fall asleep in their bed while watching porn. It kind of speeds up the process and sex talks with them;for the milkshake route I have no idea mostly because I always play as a guy and the shakes aren't really for guys to consume unless you just snag them for yourself