It depends on a lot of factors, for example in my country you can’t just start a union - they need government approval and there’s basically one allowed per company/“single-employer” enterprise. It’s basically bullshit designed to stop workers organising and making it illegal to do it without going through the official channels and a lot of those unions are captured by the companies and work against it’s members interests.

But in generalist terms the first step is to start talking to your co-workers, create an atmosphere where they feel comfortable talking about their workplace grievances; Discuss wages, comment on safety concerns, complain about unrealistic demands, etc. Don’t even mention the word union, until you’ve got a lot of people happy to just hang out and talk shit. Once you’ve got a bit of mass, you can work on bringing up some demands to your bosses as a collective.