Now the fun part, paint a line of highlights with thick opake perminent white gouache. Make thin, iregular lines along the reflecting surface. Study where the light falls on the real subject (life or photograff). If you need to make it up, try placing it between the centerlight and core shadow, a little closer to the center light. A little highlight goes a long way. It is easy to overdo it so just add a suggestion, then stop.