Hey, sorry for the slow reply! Have you chosen a mistress? if so, each meeting with her (i.e. each big chapter event) increases your feminized level by 1 point. At first you wouldn't wanna buy any girly clothes, but with each event you get a little more curious ...
That said, I'm still to add in the full 'present as a girl' mechanics as that's a whole other layer of ... fun. A little like the chastity stuff, there's a bunch of new reactions/descriptions/etc that need to be worked on once you're totally passing as a girl. Hope that all makes sense!