Feed them only what they clean up in 10 minutes. Don't feed raw potato peels. Don't feed anything spoiled or rotten. Moistening their food may stimulate their appetite. Hay chaff is a nutritious supplement.
Minerals are available through kelp.
Available excess milk or whey in modest amounts make good supplements as well. In the case of mashed grains, buy only what you will need for 2 or 3 weeks, as chickens don't like stale food any more than you do, and the nutritional value of their food reduces with time. Be systematic about recording your observations. Your birds will teach you a lot about their needs. They do well on a free-range pasture, choosing from a variety of plants and insects to balance their diet.