Hmmm.. I was hoping nobody would ask that.
Reason being it's pretty subjective and while most things bees are easy enough to explain I think this one is about "reading" the hive and conditions.
When I've failed totally at it, is putting a cell in a hive with a young, or at least, very active queen, and at the wrong time of year.
In one place I used to work, any natural supersedure was always done st the same time of year, being early fall, as the flow was ending and just before we took the honey off. In that area it was simple enough to put a close to hatching queen cell into a hive with a 2 year old queen, at the same time we took the honey off, and get a fairly high strike rate. We put the cell right at the top of the brood nest between the top bars, but used a cell protector.
Whenever I've had virgins hatch in one of my cell finisher hives, what I do is shake everything down below the excluder, then put the hive back together and continue to use it as a finisher. But this nearly always results in supersedure there will be 2 queens for a while and the virgin (now mated) will eventually take over.
So I know that's not really a straight answer, in fact, I don't think I have a full understanding of what it takes to be fully successful at requeening with cells. But i think the basics are working in tune with what the bees are wanting to do anyway. It is nessecary to protect the cell though, for some reason even though bees are planning to supersede they will still often tear the cell down. Also, if the cell goes in and hatches immediately, it is more likely the virgin will be killed than if the cell has been in a couple of days before it hatches.
Sorry can't give an exact formula for success, those are a few rambling thoughts. But I've also found, because I've worked bees in several different places, that what works in one place may not work in another. I think people have to figure out what works locally. The good thing, is it requires very little investment, just raising some cells. 2 guys working together, one opening hives and one placing the cells, can do several hundred hives in a day.