When drawing building and other structures in your scene, remember this simple rule of 2 Point Perspective: The top and bottom edges of walls, doors, and windows all point either up or down towards the horizon, depending on where they sit in your scene. Edges that are above the horizon slope downward; edges that are below the horizon slope upward. These both converge at the horizon line in a spot called the Vanishing Point.

To capture this accurately, first establish the horizon in the landscape and draw it lightly across the paper. Now follow an obvious roofline or other edge towards that line. Place a dot there. Do so in each of the 2 directions - you will have a small dot on the horizon line at each side of the paper. Now all of the top and bottom edges should aim directly at those points, as shown in the example above. Mouse over image to see the basic framework drawn out in daigram.