When unionising, my main piece of advice is to let the other person talk themselves into it.
Literally everyone hates their landlord, so this isn't hard. (If you're a landlord reading this, yes, everyone hates you.) However, people often find the idea of doing anything about it to be frightening. So, at first you have to normalise the idea of talking to one another about what a scabrous piece of shit the landlord is. Ease people into it. Let them think they're coming up with ideas.