Here are some of my thoughts...I'll add more later...
1. Have a reasonable explanation for your medfet, I.e. tell your wife/girlfriend that they are the most beautiful/sexy woman in the world, and that you would like to appreciate their beauty and practice self control by doing a play exam on them. Explain to them that your medfet involves them, not just some random woman.
2. Start out slow: Tell her that you were always curious what her vagina looked like with a speculum inserted, that you wanted to see what a Dr sees. Buy a speculum, and one night while you are giving her oral sex, ask if you could put it in, after which you continue to give her oral until she orgasms. She will then remember having a speculum in while orgasming. You could also massage her breasts like a breast exam while you give her oral, or wear gloves while you massage her pussy. Tell her you enjoyed the experience and you'd like to do it again once in a while.
3. Set up role playing scenarios that are fun and exciting for both of you. Make a "pretend" appointment so you both have something to look forward to, and make every exam different so neither of you know what to expect. I posted more ideas about this under another thread.
4. Set boundaries in advance and make sure she agrees to whatever you want to try. I.e. if she doesn't like the feeling of a speculum, you could skip over that part of the exam, or use a dildo/vibrator instead. The important thing is to make it a safe and pleasurable experience for her, otherwise she will never want to try it again.
5. Talk or text her scenarios, and tell her how much you are turned on. Sometimes talking through something in advance removes any inhibitions in addition to building feelings of excitement.
6. Don't overdo it. Everyone has their limit, so make sure that you enjoy your fetish sparingly. You want to have regular lovemaking sessions, as well as help them satisfy their fantasies, so that everytime you have sex it's not associated with medfet.
7. Thank her and tell her how much you appreciate her helping to satisfy her fantasy. Make sure you reciprocate by doing nice things for her as well: I.e. give her a 1 hour body rub after her play exam.
8. Work some legitimate purpose into any play exams: I.e. really do a monthly breast exam, or do a full body skin exam, etc.