A lot of reading and taking shit apart. I started with the really small easy stuff like changing tires etc as a kid and worked up to harder stuff as I learned how to use the tools. And tools are important.
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The D3V
I just youtube everything.
Didn't even think about all the technology nowadays.
This is an excellent resource. Granted not everything on the NET is true, it definitely helps to at least know a little about the job before you dive in. Us old farts on the site didn't have that growing up.
I think my real knowledge base came from growing up in the 80's where we all had BMX bikes and were always braking parts at the track and fixing them.