Grew up in the country, always had a few feral every year. I did take one in particular when seh was tiny, barely big enough to be away from mom, took her to my room, gave her a litter box, and food and water, and a good hiding spot, slowly over the next two weeks she came around, I spent a lot of time being quiet, and waiting. My MIL actually ended up adpoting her. She was a great cat, very sweet, and was a great hunter. She was healthy and lived to be about 14.
I have tried, not to that extent with others, but without so much luck. I think the trick is bringing them in when they are still pretty young.
The one I have now was a dump off, found in the woods by the local no kill shelter, and she is one mean fiesty cat. She is sweet, but for very short periods. Not sure if that's just her, or if they were truly feral.