In addition, when James suggests spying on the girls in Rachel's room, agree to it. When you have the chance to spy on her in the shower, do so, and choose to stroke yourself when you see her. Later, when she sees you masturbating at your computer, don't stop. Let her watch. Over breakfast, when she asks if you've ever had sex, be honest (don't boast) and be sure to question her about her virginity, rather than just reassuring her. All these choices will get her corruption score up to six or seven, which is what you'll need for the relationship to start after the movie (be sure to tell her it was arousing). That night, she will come to your room, looking for company, and you'll be on your way.
There are also walk-throughs you can find online.