If I were doing it by hand, I would begin by tracing out all of the ridge lines and the trough lines. Some of those ridge lines are pretty obvious--basically, you are starting with your pen at the peak of the mountain and then drawing a line to one of the "spikes" in the topo-line, then to the corresponding spike in the next outer-most topo-line, and so on. When you draw the trough lines, you are probably drawing them from the outside-in, following one divot in the topo-line to the next divot. Make sure you keep the ridges and troughs distinct for yourself.
The magic comes in when you begin shading and highlighting. Pick a direction for the light, then shade the "backside" of the ridge line. Do this gently at first, and make sure that the darkest shading is right along the ridge line itself. When highlighting, put down a light color on the side of the ridge that faces the light. Again, the brightest intensity of the light should be right along the ridge line, just next to the darkest region of shadow. For the troughs, reverse this advice.
That said, I'm sure others here will have more "automatic" methods that allow the computer to do most of the shading work. I'd be eager to hear about those methods as well.