This is a rough guide to the way I construct my paintings using photoshop.
The tools I use are brush, smudge, eyedropper for picking colours up, dodge and burn, and fill for the two background layers.
The brush tool is set to 100% opacity for blocking in colour, 40% opacity for the sketch and some details such as hair strands, 20% opacity for some detail and hair strands, 10% opacity for building detail, 2%-5% opacity for building flesh colour.
Flesh I have in two layers, one layer for the face, another for the neck, etc. Helps to keep the face and jawline in tact.
I usually have two layers for the hair also, upper and lower. The Upper layer is for hair in front of the face, clothing etc. the Lower layer sits below the flesh layer and contains hair that sits at the back or is partially hidden by the face. This is also the layer I put loose stray hairs on, giving the outline a much more realistic look....hopefully!!
The 'Lucius' portrait featured is still being worked on but I use exactly the same technique on all the computer based portraits I do.