What is a good process to go through to convince them of the benefits of unionism?
You need to show them the benefit of doing so.
Before you take any action, individually or not, it would be good to get together and talk about it. Because you are perfectly right that people in a group have more power than each of them individually.
The first step would be to invite them for a beer (or barbecue) after hours and talk about what you all want from your employer.
When you know what you want, it's time to talk about how to reach it. If you think a union is the way to go, you will need to present the benefits of a union over not joining a union. Maybe you can get a union representative to join your second meeting and explain those benefits.
A word of warning: you have not mentioned any benefit of joining a union here. You have not convinced anyone here. Be prepared that you need more facts to convince any of your colleagues.
And in the end, be prepared to do this without joining a union if your colleagues do not want to do so. Joining a union is a means to an end, don't lose sight of your actual goal over choosing a tool to reach it.