Here is a good way to approach it that worked well for me. I did this with my girlfriend, who is now my wife, so it seems to have worked well.... I spent some time talking to her about ways to make our sex life adventerous, and would talk about many topics. I told her I liked to talk and tell stories, but talk is of course much different than doing.. Anyhow, I would talk about girl-on-girl, sex in public, ...., and of course diapers. When the topic of diapers came up, I explained that I "read" an article about how some women enjoyed being diapered, and that many coupes were into it. I did this to gauge her reaction, and make sure that she would be open to the next step. About a week later, as we are starting to make out, I explained to her that I got something for her that I thought she would enjoy. I ended up diapering her and while she giggled and laughed a bit, she seemed OK with it (since it was done in a playful and fun way). Interesting thing is that she said "if I'm going to wear one, you're going to as well"... BINGO!!!
I let her diaper me in a fun and playful way and acted like it wasn't that big of a turn-on. We did this a few times, and eventually she got tired of wearing diapers herself, but was fine with me wearing them. I told her that as odd as it seemed, I actually found them comfortable and relaxing. To this day, I wear them on occasion and my wife is fine with them, so long as I don't choose to wear them in lieu of sex with her (which is not a problem).
Hope this helps....