If you download the walkthrough file from the mega link, the Katie section lists many of the events that change her dominance value. (It lists both increasing and decreasing it.) I'd list those here, but I don't want to spoil anything. In any case, once you get her dominance value to -2 (do two events that reduce it without any that increase it), that will make her submissive for most if not all scenes. That will also unlock her first submissive date. (Each date after requires another 2 points making her submissive, but each date provides one. There are four in total.) I'll try and update the walkthrough soon, as it definitely doesn't list every scene that can change Katie's dominance value.