Here's a very simple illustration, showing the fundamental way of rendering in pencil. The "slinky" stroke (as Anthony Ryder calls it, in his outstanding Life Drawing book) is a unique way to describe a certain kind of pencil stroke. Ryder calls it a "slinky" because it goes back and forth, back and forth, just like that childhood toy, the Slinky. This is the basis for much of the rendering and shading in pencil and pen. It's as simple as that! If you are unfamiliar with drawing with this stroke, practice it in your sketchbook. It's fun!