I really only feel comfortable answering your first question: If you were already with someone, how did you approach them with the polyamory subject? If instead you were single but got into a poly, how did you get into said relationship?
I approached my husband and told him I was drowning in our marriage as it is. Explained that I didn't want to leave him but I couldn't continue this way. Through talking we realized that he would spend some time learning about it and processing it and I gave him his time and we started off where it was just sexual for me, but then I realized I was more poly where I could have feelings for more than one person and he grew with me in that area. I guess I'd just suggest open and honest, it's the best way to go. Be prepared however if your partner just can't get on the same boat you are on. I always say I don't know what would have happened if he wasn't as amazing and open minded as he is. Good luck