If there isn't one in your workplace, start one. You need to find two other people and then you start meeting regularly, perhaps biweekly, and learn how to appoint the chairperson of that meeting and a secretary.
Once meetings and documentation is working, decide on a small issue that you know that more people in the workplace are annoyed by and would be an easy fix for the organisation. Then you write to the relevant boss and present the demand. Either they fix it voluntarily, or you wait a while and then you write to them again and add a demand to have a meeting about the issue.
Escalate slowly and methodically, and when you get the win, lay low for a bit and use it to recruit. Make all decisions by consensus in the beginning, don't start with majority decisions until most or all of the workers in the department have joined. Always escalate slowly and gradually, until you have the power to shut the entire organisation down on a short notice.
Act in a way that makes your union predictable to the bosses. Corporate management is easily confused and you'll prefer them to negotiate over lashing out wildly. When people are inevitably fired, keep in contact with them, both personally and through routines you establish in the union. Some people are unsuited for negotiation, documentation and so on but are good at doing phone calls and checking up on previous members and reporting back.
Focus entirely on things that are important to make the daily work less dangerous, more convenient, more rewarding. Small steps, low hanging fruit first, only one issue at a time. When you have a success, send a note about it to some local labour friendly organisation or publication.
Don't waste time on identity politics, everyone in the workplace who isn't a boss is a worker regardless of what they do or how they're compensated, the union does not need a name or bank account until you have federated with quite a large amount of people. You don't need to register some association to have meetings and make decisions, that's for when you have like a few hundred people and want to throw parties and have conferences and things like renting a place requires you to register with some bureaucracy.