I'm going to start with the basics. I think the following are most important for most femme and non-femme people.
Femme means you. Not everyone wants to be treated like a man. Many people are not attracted to males. You will never look like a man. You will always have a feminine side to you, no matter what.
Femme is not limited to just femme. You can be a man, an androgynous woman, a lesbian, transgendered person, or any gender that fits into your body.
Femme is not a word. If you want to use that word, then feel free. But don't try to use it, and don't use it to describe yourself. Don't try to make yourself look different. You don't have to change your name, or wear a dress to school, or do anything to fit a more femme definition of femme, other than being more feminine than other people.
The word is not for men to use to describe women, and I won't. There are lots of other words for women that aren't about men, like "feminist," "activist," and "champion."
I don't understand your confusion with the word "feminism." Feminism is a political movement for women. It's a movement for women to not be treated like a thing.