Ok, here's how it works.
You start with an elipse, filled with a flesh tone. Use a soft brush tool set to airbrush and screen. Take the orange color and start to color a nose, lips, cheek, etc. Use a photo for reference if you need, and just "indicate" the shapes, don't worry about making things precise and don't worry if your proportions are a little bit wonky. This should help you to see where light and shadow go on a face, without relying on the shapes of noses, eyes, mouths and cheeks.
I've drawn it in steps, so you can see how to go through the progression, I've also added a couple of advanced versions at the bottom. It seems like a kind of goofy exercise, especially at first, but it really helped me understand how to light a face.