It's all about talking. But mostly it's about listening. And taking risks.
The risk is exposing yourself to possible rejection and/or criticism.
That's the worst part.
While everyone is different, the typical process of asking for something, be it an enema or a glass of water is to consider the other person's situation at the time.
For this more sensitive topic, I suggest setting down in a neutral place with the idea of talking for a short specified period of time, say 15 minutes maximum.
Explain to your significant other that this is a sensitive and serious request, and ask that she frequently give you feedback on what you are saying to make sure that she understands what you are saying.
Then tell her about your feelings. Why this is so important to you. The excitement it provides, the pleasure, whatever. Make sure she fully understands.
Then ask her to share her feelings about the subject. Gently ask her to explain her feelings.
Talk about how doing new things, things that may seem forbidden or strange can be either exciting or scary.
Discuss the taboos associated with enemas, with bowel movements, with toileting.
And then maybe introduce the fact that the anus and rectum share the same innervation as the genitals, that the anus and rectum muscles contract during orgasm, and that anal exploration can be very pleasurable.
But this talk comes wayyyyyy after you first discuss hesitations and concerns etc.
And of course, unlike the stories, which are all aimed at fantasy excitements, remind her that this course will be taken very slowly with her leading the way. And make sure that both she and you are very clear on the safe word that means everything comes to a complete stop.
Good luck!
Larry