For me Its quite simple. My brain decided it. Literally. The days I found out I'm trans, my brain would do this weird thing, where I could think about completely different stuff and suddenly my now chosen name would "fly" into my mental Field of View, like an asteroid in star wars or so. I told that name a close friend and 4 weeks later when we went to the club together I told him, that I will stick with it, because I didnt had a better idea (and it was the only name I really felt comfortable with).
That honestly took me over 6 months. I tried a number of names on for size, but none felt quite right. The ones that felt right were in use by close family or friends. The one I really wanted is used by both my ex-wifes, so that would've been weird for the kids even if they're not kids anymore.
It was much easier picking out names for the kids actually. Otherwise I followed the advice I got to take a name that is sorta common in my age group so I went through all the lists multiple times until I decided. I now officially have a girl's name, even if I'm nowhere near passing. It isn't as bad in my country as in many others and I must say it went a lot better than I expected, but I'm still proud of myself. 😊