For reference, I am not "out" to anyone besides my mom (and the guy I'm about to tell you about), and I'm not on T or anything yet. I only bind and dress male.
When I came out to my boyfriend, I did it before we started dating, when we were in a very flirty stage of friendship. What I did was "test the waters" by telling him that I liked to be androgynous and that I am "very uncomfortable with the fact that I am female." He was fine with that, so as we continued to get to know each other, I tried to make my opinion of my body as clear as I could.
He asked me early on if I would be his girlfriend, and I told him we needed to stay friends longer. After getting to know each other better, I determined that he was very open-minded (he's even pansexual), and when he asked me again to be his girlfriend, I came out to him. I told him I couldn't be his girlfriend because I'm not a girl. ;P We've been together for about a month.
I think it's a good idea to tell the person after you at least have some idea of what their response will be. I felt pretty confident coming out to my boy after discovering that he is pansexual and very liberal-minded. I wouldn't just come out to someone just because you find them attractive, but at least get to it before you kiss...