Studying for literature class, offering Ellen the cinnamon roll, telling her she looks young, telling her you're dying, telling Gunner you're looking for someone mature are some of the factors that can unlock her route. Then go to the literature seminar and stay behind and accept her drink. Be careful to avoid the choices that heavily bias you toward one character's route, like calling Claire fat.
I absolutely will as soon as I figure it out, ha ha. I'm trying different combinations with the conversations to try and "balance" it so I don't end up locked in one, but for some reason, it still gives me Rose every time. Its very subtle, difficult to determine which stuff is really impactful and what isn't and how.