It should be obvious to everyone just how important provision of the proper learning environment by these adults is. The actual teaching of the child is only the smallest part of what the child learns. A loving child will be happy to comply. Some experts advise exaggerating the lip movements as parents speak to their children, but that's up to them. What I would recommend is that parents speak often and clearly to the child.
Good pronunciation is important, as is involving the child in everyday activities.
Involve the child and enjoy the quality time that comes with doing things with the child. I also don't recommend the use of simple words - use the words you normally use, and expand the child's vocabulary. Just talk normally and it will all work out just fine - provided you use good speech. The child will probably start trying to say words around one year of age, but will have stored lots of words before that. Don't worry about when the child starts to talk - the vocal cords get hooked up when they get hooked up.