One of the last horses I re-trained was a gelding that had run wild for 3 years. He as afraid of all things human.. tho he had been halter broke. I was told he had been ridden but the first time I tried him (about 2 months after I got him) I found no evidence of it (no steering and no brakes.. so we went back to retraining as if he had never been handled).
What I did was get Butch in a stall and then the ONLY time he ate was when I was there. He saw no other horses.. nothing.. unless I was there. He had NO idea what grain was (but got the hang of it very fast LOL). He could only eat grain if I held the bucket. He could only eat hay when I was standing next to the pile. When I left, the food went with me.
I think he made the connection of "Human shows up = food" in about 3 days. In a week he was nickering for me. At that time I haltered him and found he was very nicely halter broke.. but if he had not been then I would have held the grain bucket and rubbed the halter over his head and neck whiel he was eating.. and started that way.
I do not know if my method was over kill or not. I will say that Butch (who was Belgian and QH cross.. and a lovely flaxen maned sorrel) turned into one of the best horses I ever had the pleasure to ride and own. He was not beautiful.. not a dressage prospect.. but he worked cattle for me and learned to work with my dog to move cows. He was reliable.. and that from a horse who cowered on the far side of the stall when I changed his water bucket.