Ask HN: How to learn to create/draw mental models for programming?

I've been programming all my life and creating drawings/diagrams of anything comes very natural to me. While this happens all the time in my head, I still grab a pen and paper sometimes when the mental overhead becomes too much. Currently, I'm struggling with a junior colleague. He comes from a different background (sales). Adjusting existing code, fixing and improving features is not a problem for him, but as soon as something becomes more complex he struggles a lot - even simply drawing it on a paper completely blocks him. I'm looking for advice or resources that can help him improve this skill.