If you want your child to have good manners, you need to role model good manners. For example, as it is extremely rude to talk negatively about someone, especially in front of them, don’t talk negatively about your child, especially not in the third person (s/he is so …!) and in their presence.
Acknowledge their presence and social participation as a human being. Involve them in conversation. Do not reply for them when someone asks them a question, or asks a question about them in their presence. Encourage them to talk themselves to friends and family of all ages (but do have a word with them about not just assuming that strangers are all nice).
Expect them to express the basic ‘hello’ ‘goodbye’ ‘please’ ‘thank you’ and ‘sorry’– don’t do it for them! And, most importantly, request kindly that they look at people in the eyes when they talk to them. Do the same when you talk to them (if your child is small, you may need to squat, bend, or gently lift them up to your eye level).
If you see your child struggle with looking at people when they talk to them, choose a time you and him/her are alone to play what I call the ‘disconnection’ game. Take turns in talking to each other (for example about what you want for dinner, or what you’d like to do at the week/end, or during your next vacation) with the listener first making eye contact while listening attentively, then looking down or elsewhere, acting distracted. Have them experience and voice what it feels like to listen to someone when you look at them versus when you don’t, and what it feels like to talk to someone when they look at you versus when they don’t. Make the game fun! If your child acts stubborn and says it feels the same, do not worry, deep down they know it’s not true. With time, the game will still have its desired effect. This game can be done with smiling versus frowning, good posture versus bad posture, sucking thumb or by biting nails versus not, etc. Enjoy!