If your pencil strokes show through on your final drawing, they can help suggest the surface planes of your subject, adding depth and dimension. Energetic and loose pencil strokes are dynamic and interesting. I like to see an element of the hand of the artist in the final drawing. Experiment with bold linework but do not use it as a coverup for inaccurate observation. One way of using line direction to show the orientation of the planes of your subject is to make your shading lines in the same direction that water would flow if it were on the surface of the object. The direction of these lines change when you come to a new plane, except in the case of vertical planes (all vertical planes have vertical lines as in the cube). A change in plane also means a change in value (dark to light). The more abrupt the change in value, the sharper the edge between two planes. For more on this, see How to Draw Rocks. You can also use linework to suggest a sphere. Use latitude or longitude lines to suggest a spherical shape. Study the way the ends of these lines tuck around the back side of the sphere. On this mountain lion skull, notice how the direction of the linework and changes in shading value suggest changes in the planes of the skull. Click on the image to enlarge. Notice how I use these lines on the lower jaw and the zygomatic arch (cheek bone). Click on the first image below to start a step-by-step slideshow of my process drawing a weathered bobcat skull. Notice how bold linework adds interest and suggests the planes of the skull.
I start with a loose line to encapsulate the shape of the skull. I focus on its length compared with its height.
Here I indicate gross structures of the skull, the ball of the brain case and the slope of the face.
The centerline of the skull is a very important orientation line. Its angle changes as it moves from the brain case to the face.
Parallel lines will help you keep the shapes of major parts of the skull (eyes, nose, cheek bones etc.).symmetrical.
The nasal bones make a big flat plane down the front of the face.
The orbits of the eyes go on either side. The parallel lines help keep them the same size. The orbit on the far side is turned away from you and so is foreshortened. Check your proportions before moving on.
A few more lines, indicating the location of the zygomatic arches (cheekbones) and other important features.
There are many ways you could break down this skull into primary shapes. Ten artists would do this ten different ways. Experiment with creating structure lines that work for you.
Using the framework as a guide, I draw the structures of the skull with a 2B graphite pencil.
Once I like the basic structure, I reinforce and enhance some lines to give a better sense of depth. Bold lines tend to pop forward, light lines tend to recede.
Now I indicate volume and planes with shadows. Notice how the angle of the pencil strokes in the shading suggests the orientation of the planes.
The last step in a graphite drawing is to add detail and texture. Notice that I show texture on the edge between light and shadow. I indicate more detail on those parts of the skull that are closer to the viewer. Also notice how a little erasing (inside the orbit and on the bottom edge of the zygomatic arch) add the glow of reflected light.
You could stop with just a graphite drawing. Here I chose to continue with mixed media. I painted in the shadows with a purple gray mixture (mostly Daniel Smith Shadow Violet).
Once the shadows were dry, I overpainted them with a warm yellow ochre.
You get interesting results by using transparent paint in the shadows, but opaque paint in the center light and highlights. Here I used Permanent White Gouache in the light struck regions.
Again, details and texture are the last step. A little goes a long way. Stop before you think the drawing is done.
One way of using line direction to show the orientation of the planes of your subject is to make your shading lines in the same direction that water would flow if it were on the surface of the object. The direction of these lines change when you come to a new plane, except in the case of vertical planes (all vertical planes have vertical lines as in the cube). A change in plane also means a change in value (dark to light). The more abrupt the change in value, the sharper the edge between two planes. For more on this, see How to Draw Rocks. 
You can also use linework to suggest a sphere. Use latitude or longitude lines to suggest a spherical shape. Study the way the ends of these lines tuck around the back side of the sphere. 
On this mountain lion skull, notice how the direction of the linework and changes in shading value suggest changes in the planes of the skull. Click on the image to enlarge. Notice how I use these lines on the lower jaw and the zygomatic arch (cheek bone). Click on the first image below to start a step-by-step slideshow of my process drawing a weathered bobcat skull. Notice how bold linework adds interest and suggests the planes of the skull.
