I have vaginally fisted ALL my recent patients, including those who've not had children. I am told I have very big hands (and I do); there are several things I've observed...
1. Don't say the word, "Fist." Just gently begin stretching. Your patient will tell you if she has had enough.
2. Use LOTS of lube. (should go without saying, but...).
3. Be extremely gentle as you open your patient's vagina with your hand.
4. Stop if there's a lot of pain.
5. Massage the clitoris and other erogenous areas while you do it.
6. In all cases, doing this procedure has resulted in multiple massive orgasms for the patient.
Of course, if the bowels are loaded up, you must give a good cleansing enema first (I normally do enemas till clear). There is only so much room inside, and you will be taking most of it with your hand.
Because I never use the word, but just do it, I have not had any recent patients "tense up;" instead, they all just appear to "enjoy the ride."
Couple caveats: fisting can be dangerous in older women. Once a woman reaches menopause, the walls of the uterus thin out. Be careful.
Gloves really help, since they can be made "slicker" with lube than your hand.
Don't try to use your fist too much like a penis (movement). You can bruise sensitive tissues, especially if the patient's not used to it. "Let your fingers do the walking" with your other hand, and massage her clitoris. You will get excellent orgasms.
It's fun for both, and especially for the girl, who, as I said, normally has multiple massive orgasms (if she's relaxed, and trusts you a bit).
"now just lie back and relax, dear, you'll feel a bit of pressure. Let me know if you feel pain..."