What I found in my practices, it is that the best way is just to ask correct questions, so they will try finding answers themselves. Especially efficient with women, as they know themselves and their bodies much better [than me or other men], while men sometimes just answering "I guess it is something I don't know, ask transes, don't bother me anymore". Questions like "feeling like a woman - how it is?" and then "what is 'woman' then in that feeling?" and similar. And saying "I don't know the answer. Do you? Can you help me in this?". If "women is gender stereotype, then butch women are not women? Your granny is not menstruates anymore - she is not women anymore too and transmen?". As most of gender theory and ideology statements are based on nothing, and if you start questioning those statements - whole theory shatters. And so on, just don't be way too active and always insist on answers, so if you hear something vague, repeat question or ask to specify. "I want to be your ally and understand you, so please explain". Obviously surroundings and country culture will play big role too, and do not forget that teens tend to take stuff as personal attacks if it is a bit too harsh or pushing. And introduce her to the idea that being woman is fine, nothing bad in it, that oppressions will be still happening even when masquerading as men, because they are sex based and not gender based, that women can be different, and playing with cars will not makes you "wrong woman or man inside" - and you can show it on your own example, or name great women in science or engineering, like Curie, so it will normalize idea that women can be different and like "boy stuff" while still staying women.