If you like to draw landscapes or buildings and want to give them a realistic (not flattened) look, you have to learn how to apply perspective to the objects. It involves knowing what point of view you want the reader to see the object from and how far into the distance the drawing goes. The main thing to remember is that as things get farther away, they look smaller, but the angle at which they fade is what will give your drawings a realistic, three-dimensional effect.
One Point Perspective
One point perspective is usually used when the object's front is facing the viewer. In this type of drawing the horizontal and vertical lines will be horizontal and vertical respectively in the drawing, and lines getting farther from the viewer will have an angle towards what is called a "Vanishing Point." Click on any of the following images to get a closer look.
Determine the horizon in your drawing. Draw a horizontal line for the horizon with a hard pencil. The horizon lines determines how far the viewer can see based on the terrain and the distance of the viewer from the ground. 
Choose the vanishing point. This is what will determine the effect of the perspective. As a reference, the most basic vanishing point would be located in the center of the paper horizontally and over the horizon line. If you set the escape point to the right, the drawing will look as if the point of view is moved to the left of the objects. The vanishing point for some objects can also be above or below the horizon lines depending on the inclination of the planes in respect to the ground. 
Sketch the main objects. 
Take care to draw all the horizontal and vertical lines perfectly horizontal and vertical. 
The lines that start near the point of view and get farther should be drawn towards the selected vanishing point. This will give the perspective effect.
Give detail to your drawing respecting the proportions dictated by the reference lines you sketched before.
Two Point Perspective
Two point perspective or perspective with two vanishing points is used when the objects' corners are facing the viewer. This method is perfect for drawing isometric objects.
Determine the horizon in your drawing. Sketch a horizontal line for the horizon as in the first method. - Determine the point of view, which is the approximate location of the eye of the person viewing the drawing. This point may be located below the bottom of the paper (outside the paper.) You don't need to actually mark the point.
Determine your first vanishing point. The usual method for this is to sketch a first line from the point of view in a 60 degree angle to the left and mark your vanishing point where this line intersects the horizon. 
Determine your second vanishing point. For this you will draw a second line from the point of view in a 30 degree angle to the right. Again, the vanishing point will be the intersection of this line and the horizon. The 60 and 30 degree angles can be different, but the angle between the lines that go from the eye of the observer to the vanishing points have to form a 90 degree angle. 
Sketch your main objects drawing the vertical lines perfectly vertical, the horizontal lines moving to the left at an angle towards the left vanishing point and the horizontal lines moving to the right at an angle towards the right vanishing point (all the horizontal lines should converge in one or the other of the vanishing points if extended far enough). 
Give detail to your drawing respecting the trends given by the horizontal lines sketched for the main objects. These lines will determine the proportion of the sizes of the objects when they are located closer or farther from the point of view. 
Make light, temporary guidelines with a ruler (shown here in green) to make sure your details are in perspective. Erase these guidelines later.
Tips
- Always start your drawings with a hard pencil. 2H pencils are recommended for this part, but you may want to use harder pencils if you want to avoid your reference lines being seen in the final drawing. Complete the drawings with a slightly softer pencil, such as HB.
A good way to practice is to visit places where you can see a structure disappearing into the horizon (railroad tracks are perfect, but be extremely careful of a train approaching from behind). Sit down and draw the structure head on; then move over 15 feet to the left (or right) and do it again. Practice drawing from various angles, always noting where the vanishing point is. 
Perspective can also be applied to block letters for a more dramatic and imposing effect.
Warnings
- Make sure you keep your hands very clean when drawing. Nothing is sadder than ruining a perfectly good drawing that took hours to get perfect just because your hands were not clean.
Things You'll Need
- Pencils of different hardnesses (2H or harder, HB, 2B or softer)
- Several sheets of paper
- At least at the beginning, a model to make your learning easier
