Lay down red, yellow, white, blue and brown on your palette to mix the flesh tones. Darker skin tones will use more blue, brown and red, while lighter skin tones will use red, yellow and white. On fair skin, use blue very sparingly (if at all). Use the paintbrush or palette knife to experiment with the proportions. Compare your mixture to the flesh tones of your subject. Paint a test swatch on your canvas, then flick your eyes back and forth between the subject and the canvas to compare.
Make the flesh color in generous portions unless your painting is small. Oil paint lasts a long time without drying. Cover your palette when you're away from your painting to preserve the paint for longer.
Mix the shadows for the flesh tones by applying blue, brown or green to a relatively small portion of the flesh tone. Only rarely---if ever---use black to create shadows.
Mix the color for the whites of the eyes. This should include mostly white, but also a very tiny amount of black, blue or yellow. The white of the eyes will have some shade to it---it will not be flat white. The same applies to the teeth.
Mix the color for the hair. Remember that blond hair has a considerable amount of brown and white in it---it should not be flat yellow. Black hair often has a blue or brown tinge to it that shows up only when light reflects off it. Don't apply black paint straight from the tube to the canvas. Instead, mix thalo blue or burnt sienna into the black paint, then lay it down on the canvas. Remember that red hair often has a considerable amount of brown or orange (unless it's bottle red, in which case---purple).
Mix the other colors involved in the painting---that is, the colors of the background and external environment. Note that it is important to integrate a subject into the painting, and one way to do this is by coordinating the colors around the subject so that they bear some relationship to each other. For example, if your subject is very fair and pale, keep the colors around the subject on the cold-side (fewer warm tones, more cool tones). Even warm tones can be cooled down by adding hints of blue or green.
In addition, try to pick up colors from the subject and place these colors elsewhere in the painting. For example, if the subject has striking blue eyes, integrate blue into the background. Or if the subject is wearing a ruby-red shirt, try to place traces of ruby red elsewhere in the background.