ha! ooof- that is a question I cannot answer! It takes practice and observation, but I can offer a few tips: don't spend too long trying to get it "just right". In my experience, watercolor works best when you move quickly with it. The other thing, and this is tricky, but try to focus on painting the light, instead of painting the object. Don't look at the contours and shapes of the subject of your painting as much as how the shadows and pools of light play off the surface. Last thing: Save your whites...work from light to dark, and don't cover up the areas you know you'll want to keep brightest at the end!