Eat
Hold food in front of your mouth, where you can see it, and keep it there. Each piece takes a few bites, and the next one in the stack comes up. You can feed a friend the same way. Keep eating when you are full and you only get a burp, then a groan.
What to eat
Ripe fruit, meat, fish and honey all work. Roasting makes fruit go further and meat and fish far further. Raw meat can turn your stomach, so cook it.
Hunger
Hunger never hurts you directly, but you stop healing, recover stamina slower, walk slower and hit weaker. Dying does not reset your food, and sleeping does not feed you.