Sketching is an action word, and you can only learn this skill by actually sketching. To give you a feel for the sketching process, numerous illustrations and simple text take you step-by -step through the process of rendering a sketch from one of my sketches (much easier than working from an actual scene).
A detailed tutorial explaining the various methods of sketching process. If you want to learn about sketching proportions, mapping values and defining values, this tutorial is a good place to start.