Hey there everything's gonna be okay. I don't have any advice really, but what I (transmasc) did was basically start off with blurting out "hey _____, you know how you feel comfortable as a woman and like you feel right and your body and even the possibility of being sexy? Well yeah I don't have that at all, I actually kinda feel really empty and deeply discomfort inside but I realized when I look at guys I kinda feel like if I was like that I could be. So yeah" something like that and it went well. Note: similar to what you said, I knew the person was generally supportive to trans people beforehand. I mean there is some crap they say that is transphobic toward me at times but it comes out of a place of ignorance and personal discomfort with/getting used to my autonomy which is a real problem but different to the problem of like, full on ideological transphobia.