Its a freehand sketch. The distance between the eyes is equal to one eye.
First of all, roughly sketch the portrait
Now start shading the eyes. Use sharp pencil to draw eyebrows. The lines of the eyebrows should be crisp. The skin is lighter then the eyebrows. So shade the skin with light and smooth pencil strokes
Start shading the eye.
Moving down the center of a face is the nose. Left side will be lit and the other in shadow (according to the light source). Draw the shadows of the nose area instead of trying to draw the nose.
Finish the nose with gentle shading, the roundness of the center of the nose, and the roundness of the nostrils. There is usually a dark shadow under the nose.
Blend it. For blending, smudge the lines very lightly and then draw light strokes with your pencil
To keep unwanted smudging at a minimum, use a scrap of paper underneath your hand as you draw. Draw the shadows under the cheek bones with light diagonal pencil strokes.
Add some more shadows on the left side of the face. Dont make it too dark because the light is highlighting that part. Always remember your light source.
keep your drawing in flow, light and broad strokes, not tight, careful strokes. Just lay in the basic shapes. Add depth to the cheeks
Darken the outline of the nose. Add some more shadows
Start shading forehead.
As light falls across the human head, you get a variety of shadows. These different shadows coincide with the different planes of the face. Note the highlights on the forehead. Shade it accordingly.
With forehead done start shading the mouth.
Applying less pressure, start laying some tone down on the upper lip. A corner of the mouth is called a pit. It is the darkest area of the mouth.
The top lip is usually darker than the bottom one. Add dark tone under the lower lip.
Start Drawing the beard. It is the most interesting part. All you have to do is draw small strokes. Some strokes should be crisp, some should be light some dark, so on.
Draw beard as shown below. For wolverine's mustache draw a few strokes and a few dark dots.
Darken the right side of the portrait.
Now Draw the hair. For more details on how to draw hair, click http://www.pxleyes.com/tutorial/traditional-drawing/2233/How-to-Draw-Realistic-Hair-with-a-Pencil.html
Shade the hands and the blades. Now it is done.