Your "approach" has to be individualized to your wife and ya'lls relationship. No two couples are the same.
If it was me, maybe after one of those evenings of "very good" sex and after she had mentioned something, I would start by just "exploring" the thoughts and ideas with her - each of you sharing your fantasies, etc. "Hey, you've mentioned several times you might enjoy being with another woman? Ever given that serious thought? Sounds kind of exciting to me...."
Or even, "Hey..you mentioned being with another woman not long ago and I got to looking around on the net and look at this site I found. Was reading it and it's really fun..." (and show her this Board!)
Maybe she even has a recipe she wants to contribute to our cookbook.
-EBF