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).
I used this advanced search tool (particularly the community impressions) to select a name appropriate to my language and culture with the vibe I’m going for. I selected 10 names I liked and had the five people closest to me vote on them. There was a clear winner.
I did the same, but I had to filter a lot more because I needed a name with variants in several languages. Ended up with 3 to vote on, and same, clear winner