Well, I haven't yet. But I might be a little further along than you.
My wife Anna is fairly conservative, but more importantly struggles to get her head around the actual concept of this. She has been quite active in fantasy play sometimes, but then another time seems totally not into it.
I'm stuck there (I have a thread on my current dilemma about whether/how to push her into the arms and/or bed of a doctor she met through work but no progress yet) but I think you could at least get to the fantasy stage from where you are.
Maybe think about starting from wherever she is. What does she like? What turns her on? Is fantasy play acceptable to her?
I think two things helped with us.
First, I got Anna to feel comfortable enough to tell me her very secret mmf/mmmf fantasy (which she kept to herself because she assumed I would hate it). I've been very slow to realise quite how useful that fantasy is - I started off by not spotting how my being involved in in a mmf or mmmf situation would be almost identical to my cuckold fantasy, just with my active involvement instead of watching or being told about it.
Second, I semi-made up - well, heavily adapted - an intense wet dream which spelt out one of my best cuckold scenarios (Anna is chatted up by the best man at a wedding and uses him to tease me until it goes much too far), but shamelessly sprinkled with details I knew would do it for her.
FWIW, although I'm not much further on than you, I would suggest starting small and not too scary. I've admitted to Anna that I love it when she gets chatted up, for instance. Especially if she shows a little interest back.
Key to anything in that direction would be reading her mind and anticipating her thought-based, emotional and physical reactions. I've had to keep assuring Anna of my complete love and lust for her; that I would never ever risk losing her; that I'm not secretly gay; and that I'm not looking for a hall pass.
For me, while I do think I would be extremely happy and horny if this happens in real life, if I can just keep the fantasy acceptable to her I think I can probably survive.
I'm also posting so the topic is bumped as I would definitely like to see some more successful cucks, and maybe some more open minded ladies, giving some advice on how to corrupt our good wives!
