I didn't even know that route existed. How do you get it?
Okay.
General:
1. Break up with Eric and don't be in another relationship.
2. Don't be too much of a bad girl. You can be a bit sexy and stuff (like you can still do the photoshoot route (even naked and risqué), and camgirling), but don't do drugs and don't sleep around too much. The route is pretty similar to the Eva love route.
3. You might (might!) want to spend a bit of your time studying so you don't flunk. Don't know if it actually matters, but eh...
Choice specific (you might not need to do all of them, I don't know):
4. When you're in Jessica's room a bit after Eric's goodbye party you'll need to have at least 10 good girl points and tell her that "You don't think it's a good idea" to do drugs.
5. When Dave is hospitalized and Jess is worried, say that: "You didn't know that Dave meant so much to her".
6. When Jess stays over at your house to avoid her parents tell her that: "That's so sad."
7. Later in the same scene she will ask if you have drugs. Tell her that: "She should quit for good".
8. Later in the same scene you can ask her questions about her sexual experiences. You can just ask her all of them, but when you ask if she has been with a girl, she will ask you if you are interested in being with a girl. Say yes. If you've already had sex with Eva tell her that you are interested in being with other women too.
9. She will then a little later in the same scene kiss you. Kiss her back.
Boom! As long as you can keep your naughty bits from being used too much, and as long as you don't enter any other relationships, you are now en route for a more pure love route between Ashley and Jessica. Just remember to always show her affection and stuff.
EDIT: It's probably worth noting that she will confess her love to you rather late in the story. A little bit after Eric's return.