First, don't even try this with Tamiya. They don't work with brushes, 20 years ago, yeah, but this formulation is pretty much useless for figures.
I start with a base flesh color, Vallejo beige red works for me. For a darker skin tone, I might add a bit of Andrea Games charred flesh. I then do the eyes with Andrea light flesh for whites and a tiny drop of blue, green or dark brown for the iris. I don't worry too much about the size at theis point, since they will be retouched with flesh at the next step.
I recently learned to look at the face in a mirror to check the iris alignment, and it works.
For 1/48, you might just do an oil wash oif van dyke brown later on, since the eye is so small, just the slit will work.
Next, I do my first shadows with a darker flesh mix, base color with more charred flesh or Andrea leather brown. This goes in the frown lines, under the chin, in the ears, along the hair line, very carefully at the sides of the nose, under the lip, in the hollow of the cheeks, in the cleft below the nose, in the eye sockets and long the edges of the adam's apple or any musculature in the neck.
Now to a first highlight by mixing the base with either sunny skintone or light flesh. This is along the edge of the chin, along the top of the lips, the ridge of the nose, at the highpoint of the cheekbones, top of the ears, to top of neck musculature and adam's apple and along the ridge above the eyes.
A second, darker shadow in the depeest recesses is next.
Final highlights with straigh light flesh at the tip of the nose, highpoint of the adam's apple, the chin and along the ridge of the forehead.
The lips get a slightly reddish base color.
Using oils, you could get by by mixing titanium swhite with burnt sienna in varyiong degrees to get just about all the tones you need. A wash in the eye sockets will give you adequate results, especially for your scale.
For everything except the base color which might be done in a #1 or #2 brush, I use a #18/0 liner. The iris is usually done with a cocktail style toothpick sharpened to a fine point.
For a tutorial on layering acryilics go to http://www.acrylicosvallejo.com/ the choose "Model color" from the drop down menu and scroll down to the bottom of the page.