I included the different stages from sketch to render, to show a little bit of my process
I included a little extra text with information on the way I approach each stage.
The finished portrait.
I use a mixture of soft cell-shading and airbrushing to give it both a more stylized, cartoony look, while having that 3D feeling.
It helps to highlight the volumes in the face in a softer way, in comparison to lines.
Polished sketch with rough values and shading.
This becomes my main reference when rendering the colors later on.
While sketching, I try to draw the lines the same way I would when inking. This helps a lot during the translation from sketch to lines.
Clean-up of the lines.
I later paint some strategic lines to soften the harshness of the clean lines against the soft rendering.
Laying down the base colors. These may be retouched during the rendering process to better match the reference pictures the client provided.