I simply told my husband about it, but that makes it sound a lot easier than it felt. I wanted to make sure I did it it at a time where we were very close, so close that I knew with certainty he would share if he had any concerns or misconceptions that he might otherwise let fester for months before disclosing for being too hesitant to ask. It helped that he's into BDSM. I at least did not have to worry as much about a vanilla partner viewing me as a freak.
But sharing a fantasy is not the same as getting him to start participating. It took a long time after that to slowly pry the topic open further. I talked casually here and there about getting a stethoscope or a doctor's coat, but I do not think he realized how serious I was about this until we were in a sexy store that mostly sold bondage stuff and there was this perfectly ordinary white lab coat. I was like, "Hey look, there's a lab coat! Wow!" and he, dear fellow, oblivious to where I was going with that, was like, "Uh-huh," and did not even pause as he went on by it. I had to ask directly for what I wanted. "I want to see what you look like in it! Would you try it on?"
He did. I had never seen him in one before, but when he saw my face, he understood! Especially when I quietly told him it made me get "instantly wet." We did not buy that particular one because it was horribly overpriced, but the incident served its purpose in getting him willing to wear a lab coat.
One day when we were lying in bed talking he was joking about me going to the doctor and he did something with his fingers imitating what doctors do, so I said, "Wow, that actually felt really good. Could you do that to me while I use the vibrator?" He agreed, and when he saw what it did to me he has been doing it ever since.
It's been a very slow process and I think we have some distance to go. I didn't want to potentially turn him off to medfet by plunging him straight into it the way he did with me and bondage. (It didn't turn me off to bondage and eventually I developed my love for it, but at first I did not think I enjoyed it because it was too much too soon.)