Children learn through reminders. We remind them to brush their teeth, pick up their toys, eat their veggies, but each day they are reminded how kind they can be by the actions of those around them.
As a parent, I'm very aware that my children watch my every move. If I'm eating candy, they're going to want some. If I ask a friend if I can help them, they will ask their friends if they need help. I speak kindly to my children and apologize if I ever find that I haven't. I watch them do the same. I always try to honor them kindly so that they understand that that is a reward itself. Lashing out at them for something I may have thought was wrong only makes them do the same. You can be direct and still be kind.
It's very often that I lose my youngest as I walk out of a store. Where is he? He's still holding the door for others. Some say thank you, and that makes him smile. He's very aware that teenage girls are the least likely to thank you, but he does it anyway. We are often at sporting events, and that means other families with smaller children are there too. My boys are very conscious of them and try to include the little kids in their games and their play. Kindness begins when you think of others. The parents of those younger children are very gracious when they tell us how kind our children are. It rewards my boys by knowing they did what they would normally do, and someone noticed it.
There is no textbook, that I know of, that can tell you how to make your child act kindly. Again, children follow what they see. If you are kind, it will fill your heart. When you see your child act the same, your heart will nearly burst, and their heart will be full.