Making a good sketch with all features in correct proportions (with or without detail) will be an easy option for this. In the following diagram I tried to show you the proportions of a human face.
Create a new layer and fill the skin color. This will be the base layer for the skin painting.
I used to do the eyes first. Use three separate layers for the eyes. Keeping the eye white layer as the base layer, I added two more layers for the iris and pupil respectively.
Use one different layer for the tear duct too.
I used an airbrush for skin painting. But it is not necessary. I too found that airbrushing is really useful while painting hair, eyebrows and eyelashes.
Set the new layers to clipping masks with the base skin layer when adding shades and highlights. Also use the blur tool with pressure 50 at the edges of the base skin layer, so that the edges are not to be too sharp. Use separate layer for nose. ( Also clipping masked to the base skin layer)
Use new layers for the lips. You will need at least 3 separate layers for this. One as the base color one for the shading and one for the highlights.
Create a new layers for adding light and shade for the face.
Use an airbrush with the following settings for creating the eyebrows.
Here comes the hardest part. Painting hair is a little tough with the mouse. But practice will take it into perfection. (I am still practicing) Use the smudge tool and warp tool ( if you are using photoshop cs2 or above) for more perfection of the hair.
These are the tools I used for creating hair,
Hard brush and airbrush for the hair lines,
Smudge tool for smoothening,
Layers with blending mode multiply and overlay for shading and lighting.
Neck and body:
The same techniques used for the face painting are used for the body too.
Here I used two separate layers (in Normal mode) for her right hand. One for base color. Another one for shading and highlighting.
Rest of the body is done in another two layers. These layers are arranged below her right hand.
In this final step I used one more layer for coloring the lip.
Also I created the background and added the final highlights for the hair.
Final result. Thanks a lot for reading