Start with the very simple - on axis small light source relative to the subject (pop-up) = very hash. Off axis light source which is large compared to the subject = soft. That's why you need light that comes from somewhere to the side of the lens and is diffused (look at macro lighting rigs here.
Translated to natural light - direct sun = harsh. High clouds = soft. The sun is bright and small relative to any subject. High clouds are natures softbox.