I'm going to trust your judgment and assume you actually do want to be girlier, rather than try to convince you otherwise. I was a tomboy for my entire childhood, but by the end of high school it had become less about not wearing a dress or makeup because I didn't want to and more about not doing those things because I lacked the confidence to do so. Maybe it's a similar thing for you. Here are some ideas:

1. Fashion-wise, develop in degrees. Don't jump straight from boxy T-shirts to ruffly dresses. For example, if you always wear graphic T-shirts and jeans, switch out the T-shirt for a blouse. You can wear a blouse and jeans for a huge variety of occasions - it may feel dressy initially, but as you get used to it you will see it just reads casual but more put-together. Your next step could be something like a) trying different fabrics of pants, like chinos, or b) trying more feminine shoes like flats or sandals (instead of, say, tennishoes for every occasion). In essence, change one aspect at a time, and don't change too quickly. If you change too much too quickly, you'll get whiplash, you won't feel like yourself, and going out in these new outfits will probably make you overly anxious. Recommendation: /r/female fashionadvice.reddit. Very friendly, very helpful, very forward thinking.

2) Similar idea for makeup. Don't try for a full face right away. Decide what's most important to you to try first. Maybe what you really want is to learn how to do eyeshadow. Just work on that for a while, then. Watch YouTube tutorials. Read reviews. Try different colors, different brands. When you've got that down, then add another piece of makeup to your look.

3) If you want more "lady-like" behavior, modern books teaching how to do that do exist (check out Amazon). I think they can be pretty interesting to read, but always do so with a grain of salt. When I read something like that, I grab on to the things I like and want to try and leave the rest. Always remember that the author is not some absolute authority; you are just getting subjective ideas from her. If reading a book like that is making you feel bad, then toss that book the fuck out!

There are of course other aspects to femininity but I'm just reeling stuff off from the top of my head. In short, I personally find that learning these skills provides a certain "social armor". It makes me feel more confident, more respected, more able to indulge in the weirder things I like without giving a fuck. Basically I find it enjoyable to wear a dress, pretty shoes and a full face of makeup, then cackle as I openly play off-the-wall, obscure video games and watch corny international dramas without embarrassment. I hope becoming girlier, if you should decide to do so, also can help you build that sort of strength.