1 basic knowledge and 1 tool we need:
1: shadow
shadow is always present, it can be erased by light, but cant be added.
2: layers
to do this we need layers, if you havent a tool that can create layers then download gimp or paint.NET, they´re freeware!
if you have no experience with layers then learn how to use, its very importent for any people who works with pics on pc.
next we need a pic we want to shade, ofc
i take this one, it has 2 suns which will be our lightsources. it should be separated in 3 layers:
1: char
2: lovely sun
3: bad sun
lovely and bad suns are transparent
we take out lovely sun and shade like normal, after that make the pic half-transparent by using eraser with 50% opacy.
note: make the shading on a separate layer!
same progress again with lovely sun
next we take out the shading and color lighted areas with red
and again with other sun, here also on separate layer!
note: shading layers (blue layers) MUST BE ABOVE the red layers!
now we have 4 colored layers: 2x shading and 2x lighted areas, display them all! you see, because of the half-transparant of blue it become purple. now we need our basic knowledge, it says:
shadow can be erased by light, but cant be added.
the purple areas are made of shadow+light, but we know shadow must be erased by light, therefore we take out any blue areas which overlap with red. if we have done this there will be only red and blue areas, no purple.
red areas: they´re lighted, you should use light color on these areas
blue areas: color it with dark color since its shadow
thats our result.