If you want chickens that are safe to handfeed there is no other way to teach them to do so carefully, but to let them try.
If they have never been handfed they will likely make mistakes and peck you to start with.
I would start training with an easily visually identifiable and low-value food treat, nothing made of meat for example, so they're not in a frenzy and not having to peck at everything to figure out what's food.
Something like holding an apple for them, or something large and obviously different to your skin in texture and shade. Later on as they learn you can reduce the size of what they're pecking at, and start to hold out little pieces for them between your finger and thumb to take.
They can and do learn from infancy onwards (under natural conditions anyway) to take food from their parent's beaks. You may notice roosters often drop what they're holding rather than hold it in their beaks to be taken from there; you can also do that. The motion of the food item being dropped attracts their attention and they learn to watch your fingers for food to fall out of them, not just attack your fingers assuming you have food or your fingers are made of food.
Vocalizing is important, tell them you have food for them when you want them to come get it, just like their parents do.
Starting young is the best bet but adults can learn too. Punishing is fairly useless for this, in most interpretations of the word, withdrawal after being pecked on the hand may teach some but not all will 'get it'.
Best wishes.
