Seeing your character in 3D

Hi Kienan! First, thank you so much for your tutorials, I've managed to improve my digital art so well lately thanks to them.

So lately, I've started teaching a few friends on how to draw with decent success. But one thing I'm struggling to explain is how to teach them to see their character in 3D to better understand how light is interacting with objects. It's something that has always come natural to me and I also learned from life drawing sessions, but that option isn't available to my friends over the internet and I'm not sure how I can teach them this skill because I just see it.

Thank you so much for your time and your tutorials!