Drawing sometimes is more of a science than an art. At its core, learning to draw is learning to see the big picture and the small details with the intention to synthesize both on paper.
Imagine creating a portrait. You start with using your perceptual skills to decompose the human face―your big picture―into individual features. You then draw basic shapes and lines to represent facial characteristics, making sure to get the proportions right, and refine your sketch. Lastly, you add light and shadow to create depth and replicate the subtle tonal values of the human face.