With low expectations for the outcome.
ha ha
I kid.
ok not really.
Let's see with my son I just kinda told him what was up. I didn't quote a verse so much as reference them, and it was all in the same conversation of "How Men and Women Ought To be". It wasn't a discussion, it was a lecture, and I didn't even camp on poly very long. In my view the best way to bring it up with children is as part of a discussion on biblical gender roles. By itself, poly is weird and not terribly applicable to a 12 year old. But in a talk about how "You're in Training to become a Man and Thou Shalt Rule Your House" it provides context and utility.
With my mom I don't really remember where i started. It was difficult because she truly did not want to discuss it, so in the end I don't think it would have mattered which verse I led with.
If I had a church I was going to, I dunno. I'd probably write a paper/manifesto and deliver it to my pastor and ask to speak with him about it. Eh. I dunno. I suppose if I was giving a talk in front of people I'd lead off with Deut 21:15-17. I like to start off by disorienting my audience. It's a character flaw that I probably won't prioritize correcting any time soon.
I don't know about what the 'best way' to initiate the conversation is, but my preferred method is to say something weird but true and see if they disbelieve me. I am mostly invested in getting people to get interested in looking into what the bible actually says for themselves, so i like to bring out stuff that they haven't heard in church to pique their interest. I am so thankful that the bible is so weird.