I honestly don't think you need much based on your profile pic.
Femininity is basically largely based upon neoteny i.e.: the younger and the more childlike (neoteny) you look, the better chances and easiness at passing as female. The older you get, the more testosterone changes your body. It doesn't stop at 18, it constantly changes the body and produces bulky bones by the time you're 30, then it goes downhill again. So, if you start before age 20, you're in the
easy zone I guess. After that, it steadily gets more problematic.
I think you will be able to pass with some clothes and makeup alone. So, I don't know if you enjoy makeup, you could try some and see what happens. Just don't do anything that makes you uncomfortable. Ask your girlfriend, maybe she wants to help.
I've had 1 year vocal training, and it's not easy to describe in a bunch of comments...

couple of things though: never try to force your vocal chords, and don't do weird voices, or you'll ruin the chords. Also: never whisper (seems a huge one according to my vocal coach as she get angry when I do it.). Then you'll have to do exercises, reading things like: days, weeks, months ever so lightly changed in pitch and stressing on nasal sounds, like vowels: OOOO, or NNNNNNNN... like: Sunday: SunnnnnnnnNNNNNdaaay.... so that you learn to talk from the mouth instead of the chest. Pitch is less important than removing the bass voice. Men are lazy, they talk in their chest, because it takes no effort. Girls talk more from their mouth. If you can get out of your chest, you're pretty much half-way there.