Ah, pesky tiny ball bearings but honestly it's not so tricky, mostly patience. Also I did welding workshops so naively confident I could actually make a frame, not a good one though! I'm a bit too lazy for all that though so... now I just ride :D

FWIW nobody should use a fixie rather than a freewheel unless they absolutely genuinely want to... because the first moment of inattention initially, being a bump on the road or just a turn they'll fall over the bike. After a few cold sweats though then it becomes automatic again, no thinking, just riding, and it's genuinely fun.