Your title is mis-leading. This isn't a fixed gear. This is a single-speed. Fixed gear means the bike doesn't stop moving unless you stop pedaling. What you built is a single-speed- it has a brake. A fixie HAS no brakes.
Salamat sa very good video, naexplain talaga yung importance of the spacers sa right adjustment sa tightening of freehub. I am experiencing problems where when I do freewheel, sumasabay yung kadena and I think this explains it...