Software: Adobe Illustrator CS version
Create New document as per required size. Go to File - Place, browse your image and place it on art board, lock it.
Let's start with face, create new layer and name it face, create a rectangle box according to face size, double click on the face layer. From the option box deselect preview mode and activate the Dim images.
Step 1: Illustration
Step 2: Illustration
Step 3: Illustration
Step 4: Illustration
Step 5: Illustration
Create background mesh for hair, and apply lighter, darker shades and color... as per original image.
Download hair brush extension from this link - http://www.adobe.com/cfusion/exchange/index.cfm?event=extensionDetail&extid=1045395.
Draw hair path using pen tool. Coloring, shining and shading all applying using transparency option, Select any hair group, go to window - transparency (Shift+F10), from the option choose soft light, color burn to get shiny effect ( depends upon background color).
Step 6: Illustration
Overall work goes on mesh texturing/shading/coloring.