Drawing from imagination
Drawing from imagination is the learnable skill of drawing a picture or scene you make up in your mind.
It is in some ways different from drawing from a real life scene or model, or from another visual reference. In drawing from a reference, the main skill needed is exact, objective observation — looking at the reference with an "artist's eye" to see its correct proportions, the shape of its contours, the relative distances and the angles of lines — and copying what one sees.
In drawing from imagination, however, there is no visual reference, except a vague idea in the artist's mind. This "mental picture" becomes gradually more and more concrete as the drawing develops. The entire creative process is a dialogue between three instances: what the "inner eye" imagines, what the hand puts down, and what the "real eye" sees when looking at the draft, which feeds back the imagination.
Useful techniques and principles
Techniques and principles that help in this process include "sketch lightly", "big things first, details last", "experiment with variants", "no line is final", "make 'big' decisions early, 'small' decisions late" and "make decisions consciously". The ideal drawing tool for this is a pencil because it is easy to erase and perfect for sketching lightly. It is generally a good idea to begin with "big" forms (ignoring the details but paying great attention to proportions, poses and arrangement) and to gradually refine the drawing. See also drawing tutorials.
When drawing digitally in an image editor, the same principles can be used. Additionally many software features aid in the development process such as the possibility to use layers, to change transparencies and to erase anything as much as wanted.
Drawing final lines from scratch (and the problems with it)
Some experienced artists have enough routine, or the gift of a very concrete visual imagination, that allows them to put down final lines (such as with ink or a marker) without the need to "develop" the drawing on paper. Children, teenagers and young amateur artists often do the same, but without possessing the rare skill to do it well, simply because they were not taught the technique of developing a drawing from imagination via various "light and sketchy" steps towards "final lines". The attempt to put down final lines from scratch then results in inferior drawings and frustration if the artist is self-critical.
All children love to draw, but many, in fact most, practically give up drawing frustrated when they reach the age of 10-15 when they see the flaws in their drawings (as compared to artwork done by professional adults or more skilled classmates) and conclude that they "can't draw". This is sad because anyone who has eyes to see can draw, and all they need is in fact to change from the children's "final lines from scratch" approach to the evolutionary technique, which unfortunatly still isn't taught in most schools at that critical age.