All forms are seen basically arise due to differences in color.
From this basic law that allows us to make various impressions on paper using a pencil.

By taking the black in terms ranging from 0% to 100%, this will be the main points to make the images in Grayscale.

A perfect image would be created if some of the points above are met.

shape & scale appropriate original shape.
Perspektif from the viewpoint
Contras (black-white differences in Grayscale)

It surely can be achieved by doing a lot of practice.

Recognize the tools used

basically
The tools used are simple, pencil and eraser.
But in actual practice it just is not enough, we must know some things, pencil itself has several categories and use custom pencil type.
To avoid a fatal mistake we have to use a pencil with a low black levels in starting work on paper.

Then the paper also has many kinds and we have to choose according to the type of image that we are doing.
In general we distinguish the paper from the paper thickness and surface type.

Paper with a smooth surface suitable for drawing objects that detail a case of drawing the human face, and be done with caution due to small mistake can make a notable impression mistake.

Paper with a rough surface is perfect for drawing objects such as animals, inanimate objects with the type of detail but have the impression of an abstract example fur.

Also in certain circumstances we sometimes need to use your own tools we have make ends meet.
This will occur at levels of proficiency.
example:

This is a self-made brush made ​​from bamboo.
This will be needed on the image with a high level of accuracy to avoid direct contact between the paper and pencil tip.

Pencil in its use is not always in normal circumstances, you may need to pencil in the form of fine powders.

Grayscale levels in 10%
It could be used as a measure in order to detect the black levels just when the shading part of the image and generate the appropriate contrast.