People have been sacked at our employer for trying that. You'd need to do it all on your own time, outside of company premises, not using any company resources (no posters or email).

On the other hand, if you can get half the people in the building to join a union, they have no choice but to recognise it.

I really don't want to work for our employer for any serious length of time. Even Widows had unions - they recognised that it keeps them honest. The only reason to oppose unionisation is if the employer plans to not be honest.