It's a simple technique for effective portrait painting with oil pastels

Step 1: Sketch Your Imagination

Draw a simple sketch of face with not sharp details. Inspire yourself from people & photographs around you.
This is a Simple women face I've imagined.

Step 2: Choose Expressions

Determine the expressions you wanna bring on the face in your sketch. This will help you to choose the colors which are essential for showing expressions.

Step 3: Start Coloring

After you've decided & imagined the face, Start coloring accordingly.

Divide the layers of color so that the lighter color is kept in background & the darker one is kept over it.
For example, here yellow is filled first, than orange. Other colors are filled in same order based on above.

Step 4: Impesto Effect

Express more effectively by repeating the colors on the area Where you wan to creat some expression.
This will thicker the color layer. Two colours can also be mixed if you wish.
Here Notice color around eyes of the women.