There were very small hints in my childhood and middle & high school, but the first real crack didn't happen until my 3rd year in college.
I finished a round of beer pong at a party. I was taking a sip of the purple drank when shaking hands with the opponent. They shook harder than anticipated and I spilt it on my white shirt. A lady friend saw the spill and offered me her black, long sleeve, form fitting shirt. I accepted.
I proceeded to receive several compliments from others at the party which I wasn't expecting, but it made me feel good. I eventually went home in that shirt (I returned it to her later that week) and when I looked in the mirror I finally saw why I was getting all of those compliments. And I agreed with them.
I then slowly experimented with more fem clothing over the years (I was in a very conservative family and town which severely hampered my progress) and the more fem I went the happier I felt about myself. Before when I'd look in the mirror as a guy I'd just think, "That's me." Nothing negative, but nothing positive. But with fem clothing I actually felt happy with what I saw staring back at me.
I didn't really see myself as trans yet; more a non binary/gender fluid. It wasn't until I finally tried E and saw what it did for me that I finally realized and accepted that I'm trans. And now I really like looking at myself in the mirror and get happy at what I see looking back at me.
I still use they/them, but I'm sure I'll eventually switch to she/her. I also won't stop anyone from using she/her when referring to me.