Two of the most important and fundamental rules of seeing things are as follows:
(a) Objects get reduced in size as they recede towards the vanishing point (where things seem to get mixed and lose their shapes), which is situated on the horizon line.
(b) The object placed in front of another will hide those parts that fall behind the first one.
Natural proportion of an object, the angle and area of view, and the perspective rules when mingled together can produce an excellent drawing.As you stand, the object in front of you will recede towards your both sides, and if go on, will vanish at some point on horizon line.Things recede and become more pointed as they go towards horizon line.
The Light, Shade and Shadows
All the objects in their natural existence have got three dimensions while our paper is only of two. As a picture or composition is a representation of its natural form, we have to create this third dimension to add some credibility to our line drawing. Only the light falling on an object can make it possible. See the example of trees given here and you will get a feel of what we mean. Light source is the main factor in this arrangement and the individual shape of the object is the other one. When light falls on an object, it creates volume and defines planes of curvature as per its individual shape. As the light goes further away from its source, it creates different tones.
Just keep these basic things in mind when shading:
a) Highlighted areas on an object are those that fall directly in front of the light source, hence the nearest two.
b) The shade forms directly opposite of the light source but varies as per the distance from it and so does it tones.
c) Shadow starts from the base of the object where there is no light at all and moves further away from the light source. As it covers the distance, the tonal value gets reduced.
d) Reflection can only be seen in the water or glasslike things. In water, the reflection of an object appears directly below and in glass, right in front.
How to Finish
Concentrate on the point of interest in the drawing. Add maximum detail in and around the place. Use thin lines with short, directional strokes. Concentrate on a small area, one at a time, and finish it. Then finish the parts falling around it. Connect these two by short or long lines as needed. Slowly build up the whole focal point like this. As you depart from this point, reduce the details. In the end, see the finished drawing from some distance, so you can grasp it as a whole. You can add some lines here and there if you like to, otherwise it is complete now. A bird of prey. It has the killing look. Cross position of the branch against the vertical lines of mountain gives the feeling of heights.