In general- you don’t. If you want to work union jobs, it’s imperative that you have relationships with people who hire union members. I have clients who have been working non-union for over 20 years. When I look at their list of contacts, everyone they know is non-union.
The good news is, at least you know what’s missing. Create a target list of people who can hire you for the types of union jobs you want. Don’t concern yourself with “how” you’re going to meet them. At this point, just become aware of who they are. Then when you have a question about how you’re going to get to them, come on back and post your question in the comments.
To your success!