Insane patience, plus common sense. Don't draw the mountains yet. Now draw the rivers. If you like fjords, like Slartibartfast, remember that they are simply drowned river valleys. Now you can draw the mountains.
Mountains are highest at the continental divide and fade off into foothills farther away from it.
If you want to be fancy, leave some little white lines to break up the shaded part, and some shaded lines on the other side. The terrain colors on my maps were added after the mountains. That good paint program you got in step 1 should know how to do this. Recommended reading Edward Tufte 's books, such as Envisioning Information , will teach you how to create images that are elegant as well as useful. It contains plenty of methodical tips for the beginning artist, and plenty of interest to the more experienced artist.