Step one. Identify the three main vertical lines of the building. I use a t-square ruler to make sure my lines are straight. The line in the center will be the longest. You can also use your pencil to "site" the length and positioning of the vertical lines.
Step two: The green line through the center is considered the horizon line. This is also your eye level view of the scene.
When you connect the vertical lines, the green line should be in the center of the angled lines. The angled lines will eventually converge in a "X" through the green line. This convergence is usually off the paper as in this example.
As you view the image below, you can see that the windows, door and roof line all follow the same lines.
Perspective is a principle of design technique that creates depth and realism to a scene. It gives the illusion of three demension on a two demension surface, the paper.