It'll be pretty hard because most companies will use a machine to do it for them..It'd be cheaper to just buy good wheels online; eagle sport, phoenix, proto sliders, they're all good wheels
You're gonna need to buy a urethane pourer, which is thousands of dollars, and a CNC machiner or another way to make cores, once again thousands, if not tens of thousands of dollars.
I just wanted to know how to do it because it doesn't say anywhere. I know it costs a lot because first you would need metal cores but I was just curious. (but my friends dad makes airplane parts and has a CNC cutting machine that uses a computer to cut so I would have cores hehe)