I've done it with an old FWB. Very easy to do all I had to do was follow her direction the first couple of times. I soon learned her body language and other cues like wetness. She was a spinner and I have relatively large hands so I could only get to 4 fingers sideways and my whole hand lined straight up and down. She loved it and always came extremely hard. Damn I miss her.
Another minor detail....that may be unique to me, is mental "confidence". If I've the time, I prefer getting a hour long colonic at a hydrotherapist near my ATF's incall. Knowing that I am completely cleaned out helps me to know that no little accidents will happen during what could be a long long attempt at fisting. If I cannot get a colonic, I'll invest the time with an enema bag and make sure I am completely clean and emptied before I venture over. Most of this is about the mental aspect, and being completely free of any potential "material" is important to me and helps me loosen up.
I probably have fisted at least 15 women in my life and they have produced the most intense orgasms I have ever witnessed for the ladies. It's not for all ladies but if you do it correctly and safely I promise she will want it more than once.