Between the ages of two and five your child will be at their most receptive to lessons about manners. This is just one reason that it is so important for you to begin working with them as early as possible. For two year olds try to keep the instruction simple. If you can teach them to say please and thank you at the appropriate times then you may consider yourself successful at teaching them age appropriate manner.
They may not yet know what the words mean, but they should be using them. As your child ages, the lessons should get more complex. A three year old can be taught not to throw food, for example, and an older child who can manipulate the utensils can be taught to properly use a knife, fork, and spoon. Just be sure to keep your lessons consistent, positive, and patient. Young children will need lots of help to learn and then remember their manners.
You can best serve them by demonstrating your own good manners and then helping them remember theirs when you catch them in a mistake. If your child forgets to use their napkin, for instance, you might ask what we use to wipe our faces. You will have more success with this kind of approach than you will have scolding them.
When your child is old enough to leave the house and spend significant parts of the day without you-at school or a friend's house maybe-you will be able to expect more of them. At this point in their development, your child is capable of displaying basic table manners, carrying on conversations, making polite greetings, accepting and making compliments, basic introductions, and respecting the feelings of others.
Now you need to be prepared to build upon that foundation. If your child hasn't quite reached this point yet then you will want to set that as a goal. Otherwise, you should begin working with your child on more sophisticated manners issues. Be prepared to continue reviewing he manners that your child has already mastered, though.
A good time to do this might be before an important meal out or a sleep over at a friend's house as you do not want your child to forget their manners under pressure. If you are there with your child, you may help them out by providing a prompt for forgotten manners, but you will not always be able to be there.
As your child gets older, they may be less willing to work on their manners with you. This is the time to really reinforce your standards for etiquette with them. They may not be willing to take on new rules, but you can still hold them to a certain standard of behavior at home and in public. Make sure that you do. This will help you to nip any developing bad habits in the bud and raise respectful and polite children instead.