Eat - bring hand up to mouth, like putting food in mouth.
More - put fingers and thumb together and bring hands together, like adding two things together.
All done, finished - shake hands palm out.
Milk - squeeze hand like milking a cow.
When to start... I start when I start giving solid foods. If baby is sitting up and looking at you, why not let them learn too?
When do you teach the signs... During meal time, some times during play if it applies (like the child wants more toys or is all done playing.
How to teach the signs... I usually start by doing the signs myself, the baby sees and hears me say the words. Then once meal time and eating are well established I usually say, "Do you want to EAT?" and do the sign. Then I answer for the baby, "Eat!" and take the baby's hand and help them do the sign. I don't worry about correct shape of hand, just pat their little hand on their mouth (for the EAT sign).
When do they learn to communicate through sign... Addie learned very early, probably about 7 or 8 months, she was really into communicating and she knew more signs than the above. It really helped her not cry and scream, as she had a tendency to cry a lot anyway. Elias has just learned "eat" "more" and "all done," he is 11 months. Honestly he woke up from his nap today and sat down to eat and said, "more" and "eat" out of the blue (he already knew "all done"). It amazes me how their little minds work. It just clicked, or he decided to give it a try.
Tips for avoiding the terrible toddler scream... When I see them getting frustrated or start to wail I always remind them that they will get what they want through communication. This may mean helping them calm down and then do the sign, then giving them what they want. Don't give in too easy. They know that wailing will get them what they want easiest, don't make it easy.
I also try to remember that the hard work you put in now will make it a million times easier when they are toddlers. They learn that their is appropriate and inappropriate communication. It also is good training that helps during tantrums and fits. We use the same thing with Addie now that she thinks whining will get her what she wants. We say, "You aren't talking the right way, I won't listen to you until you can talk in a happy way." Like with sign language we often have to model the right way, but she gets it. At the very least sign language will give your parents something to brag about, that they have the smartest grandchild ever!