The steps
Choose a black and white picture. I chose the very first picture I worked with.
Create a new layer for each color part (skin, lips, hair, eyes and background). I recommend you to begin with the face.
Now, click on the face layer and choose the brush tool and a color appropiate to the skin one's (like #EFD6CD ).
Start coloring the whole face with the color you chose. Then, switch the layer mode to Color.
Continue on doing this till coloring the whole picture.