It takes time. They need to feel comfortable with you and their surroundings.

I trained a couple of abandoned kittens by feeding them some canned cat food on a spoon and they got close enough for me to pet. (They ate dry food normally)

After several weeks, I was able to hold one and the other never came around. He preferred to be pet gently on the head and was not interested in being held.

Good luck. It may take a few months---just don't stress them out by grabbing them before they're ready. ;)