Don't be nervous. Light and shadow still apply as they do on anything else.
Paint on your "base" color. Give the face a dark wash for shadows. Then highlight overall with a medium tone. Finish by highlighting the cheekbones and nose.
For an easy reference check your local mirror to see how light falls on your own face. Also, turn the model upside-down and paint in a shadow under the chin, nose, and eye sockets.
It only takes practice and looks like you'll be getting plenty. Keep your paints THIN.
The old eye painting technique of painting a stripe through a patch of white is only one of MANY tricks to painting eyes. When I paint eyes I will try several methods til I find one that works for that particular figure. Sometimes painting them up to 7-10 times before I'm satisfied.
Make sure you get an artists pen with Archival Ink. It doesn't bleed or fade over time. And when it runs out, use it as a paintbrush. It will last you a long time before you 'll need to replace it. ;)