My wife brought it up by bringing up the topic of each of us sharing a fantasy. In the fantasy she shared it involved light bondage and spanking. A while later she showed me some items here on EF and asked my opinion. That was a good non-threatening way of bringing it up.

Usually honest communication is the best policy, in my opinion. Or gather some resources that you can give her to help her learn how to safely and competently do the things you're hoping to do. You can also look into a book called When Someone You Love is Kinky by Dossie Easton and Catherine A. Liszt available on Amazon Here.
i introduced my Mistress to BDSM and Wwe now happily embrace Oour BDSM lifestyle and play. Good luck and feel free to private message me if you have any questions or just want some support from someone who has been there.

I test waters first. Usually I will discuss a certain aspect of BDSM be it either bondage, flogging etc and try to take notice of their reaction. If the reaction is a positive one then I will approach the topic of playing.

I try to keep the conversation as comfortable and honest as possible.

Maybe watch a video like "Pretty Tied Up" together. Sometimes we try things we saw. Or maybe tied yourself up partially and wait for her to walk into the bedroom.

I am very direct person so just sit down and talk about it with her and set boundaries. That's what me and my partner did.