first you have to be a figure that she/he need. maybe it will help him/her to be more confidences, because she/he will listen and do what you ask her/him to do, and she/he will believe in every word that you say, it makes you more easier to enhance his/her confidence. and the most important thing, you should know what is her/his character, and search for how to handle it, by reading a book or a journal of personality. and, ask him/her, what makes him/her comfort. maybe my answer will help you.
I think that active listening it´s a good tool to use with them. Showing yourself as a close and sincere figure. From my pont of view, you must explain things to them in an understandable way but without lies. Improve their self-esteem with a few responsabilities that they can easily handle.