Just get into a relationship, so Lena can cheat on Ian. So don't have sex with Jeremy, when Lena has that opportunity. You can either avoid that opportunity, by not giving him a handjob during truth or dare at Ivy's apartment, or have at least one will point to resist the temptation. I prefer that, because if you're gonna cuck Ian, naturally you want Lena to lust after his best friend. She'll resist the temptation to get into a relationship with Ian, knowing full well that eventually she'll succumb anyway. Because deep down, the idea of being a cheating whore, is just so exciting.
Once you're in a relationship, Lena can start cheating. Literally the same day with Axel if you want to. But to cuck Ian you have to wait for a sex scene in Chapter 10 between Lena and Ian, that comes immediately after a scene at the park involving Holly as well. Lena will claim she doesn't feel like having sex, and offers Ian a handjob instead. You need to choose to receive the handjob, if you choose not to, they don't do anything and you don't go onto the cuck route. Next Lena will start giving Ian a handjob, and then she comments that his cock is one of the best she's ever had. At this point, you can choose to complement Lena, say nothing or query her comment about it being one of the best. If you choose to query her comment, Lena proceeds to clarify why it's not quite the best she's ever had ( because of course, Axel's giant cock still holds that accolade). Ian gets embarrassingly turned on by this revelation, and Lena begins to suspect he might have a secret fetish, that she could use to her advantage if she continues cheating on him.
From this point onwards, you're on the cuck route, and Lena won't allow Ian to fuck her after this. It's just handjobs, footjobs and the occasional blowjob from this point onwards. And lots of teasing insinuations about what she might be getting up to, with other men. So make sure she takes every opportunity to fuck as many guys as possible. Because the more she does it, the more she gets turned on by betraying Ian. The more ruthless you are, the more likely that Lena will end up forcing Ian to go along with everything, when he inevitably discovers the truth. Personally, I can't wait.