Every case is different of course, but I made friends with a pair of feral kittens. They would hang out around the house anyways, and I was feeding them wet food every day. I got them to warm up by scratching that lower back/right-in-front-of-the-tail spot while they were busy eating. I think it helped them to associate myself with good things. I would feed them and just hang out with them for 20-30 minutes a couple times a day. Eventually they’d come to me just for attention and that’s when I knew I had them. Took a good few months. Also their mother didn’t seem to be around anymore, it was always just the two of them, so they may have had a recently developed maternal hole in their lives at the time. Brought them inside after about 4-5 months and they are my excellent little buddies now.
However at some point they’re going to get too old and will never warm up to you, so you’ve got a very limited window.
My old thread about bringing in little Opal and Willa: Anyone have experience taking in feral cats?