What we did was adopt 2 older cats from the humane society that were deemed "unplaceable" basically because they were feral. Both my boys were de-manned and vaccinated by the humane society. When they got here, the humane society officer said to leave them in their crates in the barn for 2 hours, then let them out in the barn and keep them in the barn for 2-3 days with food and water. We did so, and they've stuck around since (this was over a year ago). Spots and Smokey now live in the garage in the goat hay pile, and their kitty kibble is on the front porch. They're still fairly afraid of us, but we've been getting closer and closer to them. They're terrified of my chickens too LOL.
Soft food is a way to any cats heart, and I can pet their cheeks when I have a can of soft cat food in hand. I'll be trapping them this year with the havahart and getting their shots done again.
We also ended up with 2 other cats, 1 i call mr. black and white mancat who is a tom cat that comes around and eats with the boys, I can't catch him but I have got him in the trap by accident a couple times so I might trap him and fix him in the summer. We also ended up with a super friendly girl cat I call fluffy girly cat, she was a rack of bones when she showed up on our driveway, probably 7 months old, and is now got a nice layer of fat... and may be pregnant... as soon as I can catch her I'm going to get her fixed and have her shots done.