The way i started with my then wife was to watch i.r porn together , nothing to heavy , just vwe blk guys fucking white wifes .
We would masterbate while watching , id ask her what cock she liked best , when she was really relaxed she would then open up abd tell me . At this point i would plant the seed in her mind of how good it must feel to be full . She would often orgasm rubbing her clit while looking at hung guys . Although there was much more involved with her conversion of cucking me im sure this did help .