For this part you will rely heavily on the smudge tool (with fingerpainting turned off). The idea here is to push the pixels into position. The effect varies according to the strength levels and the brush type. Hard brushes are useful for creating sharp edges and detail, whereas feathered brushes give much smoother strokes and can be used to create a cloudy texture.  
The higher the strength % the stronger it will smear. It takes practice to master, but eventually you will develop a feel for it, much like all the other techniques in this tutorial.
Stage 3: Definition
Rather than painting straight onto your picture with the airbrush tool, the idea is to dab a few spots, highlights and shadows where you need them and use the smudge tool to shape and define them. It works a lot like adding definition, except you will use smaller brushes. 
Stage 4: Detail
Definition
Detail
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