By Matt Fussell
The first thing that you will need to consider is that oil pastels are a loose medium. What I mean by this is that details may be hard to come by. You should begin by accepting this and decide on ways to suggest details with deliberate marks. For example, unless your drawing is large, eyelashes may be implied instead of drawn.
Oil pastels are meant to be mixed. So when you are drawing a face with oil pastels, don't just grab that manufactured skin tone. Instead, consider mixing the skin tones. I use a saying to help me remember which colors to use- "Red, yellow, brown, and white, that's how to mix your skin tones right." It's okay to use the manufactured skin tones in areas, just be sure to keep your colors natural-looking.
Layering your color is also important. Layering color is somewhat similar to mixing color. When you layer colors, you allow some of the color underneath to show through. This creates a depth to the drawing that is important to the illusion of realism.
The last advice I can give you is to block in your colors. I usually look at my subject find a color to match the color that I see, and put that color down in the shape that I see. Eventually the image emerges.
The following video demonstrates how to draw a portrait in oil pastels...