MaximumBunny: "Draw what you see" is the biggest BS. It's "how" to draw what you see that's important but is never covered. :/
Draw what you see is probably a simplification of something a bit more sophisticated. Certainly, observation is super important. And by that, I mean understanding what you see. We all flippin' see. But we reduce what we see to abstractions. Then we instead draw those abstractions, and that's where the problems arise.
There's a small, white, cardboard box on my desk. If I asked someone to represent it, they might draw the standard and familiar wireframe cube. (I see a box! I know how to draw a box!) The dimensions and perspective might be correct, but the drawing would still seem off. Why? Because if I look carefully, I notice that the three edges which face me are the brightest parts of the box. They're corners, and reflect light directly to me. They shine. In the drawing, those edges are black lines, but in reality, they're the brightest part of the image. They're effectively white. One of the most prominent aspects of the drawing is wrong.
That said, if I were to replace the text on the face of the box with small wavy lines, it wouldn't be nearly such an obvious visceral mismatch. So you don't need to draw exactly what you see. You need to know what you see, and decide what elements to represent most carefully. And how do you make that choice? Mostly, experience. You draw a subject; the results aren't quite what you expect; you compare what you've drawn to what you're looking at, and hopefully realize some of the differences that make the picture unsatisfying. And next time you draw an object with edges, you'll note the quality of those edges rather than simply reducing them to lines.
I am kind of an idiot, though. Ask my family.