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 started making a big list with checkboxes on it. I added things like nicknames and "boymode" nicknames where they applied. Then I went down the list and checked off the ones I didn't really like that much. Then I started saying them out loud one night, and my spouse would just say "no" to them and those were checked off the list. I have two left, now. I'm still open to suggestions from others, and I am asking people when I come out if they have suggestions. The hard part is that both of the names I like are either my spouse's initials or my mother's, and I can't stand my mother. One day I'll make the choice final, but I'm trying one of them out now. The clinic and hospital network I use has a field in my profile for "preferred name" and they actually use it, so it's in there for now.