So to do the sub route, basically just don't be nice to Luna. Don't ask her to sit in your lap if you have the option of telling her to sit. Don't tell her to sit if you can forcibly pull her into your lap. Never call her Princess, or say please. If you stick with that and generally be 'demanding' in your interactions with her, she'll end up on the sub route. Then later on, Hermione will 'interrupt' you two, and instead of Luna taking charge of the situation, it'll be Hermione in charge and she will make you 'release' Luna from your grasp. Later on, Luna will come back on her own.