(I started on the nose and the cheek as you can see in the painting - I also did a test with some skin tone in the forehead that I wasn't happy with.)
As usual, we started by mixing our colors - you want to have it all ready when you want to use it and not have to be mixing colors when you're supposed to be in the middle of painting.
For the light colors: an oilier mix, 3 parts turpentine 1 part oil (last was 4 to 1).
The reason you need it oilier has to do with how oil paintings dry.
The dark lower layer (that we put on last week) may still be "wet" under a dry feeling top.
The overlayer of new, wet oil will get sucked into a wet underlayer and become dull so give it more oil.
Same mix of color for skin as before, basically: yellow ochre and white;
same plus cadmium for brighter areas,
same plus green for darker.
Mix pure red and white (with a wee bit of ochre to dull it) for pink cheeks.
The fewer colors mixed the richer the color.
And for this top layer, go for a thicker paint - you don't want the underneath to show through (much).
You can also shade with thin lines (not sure how?) and by brushing so as to just hit the nubs of the canvas.
Painting before adding red background For brushes: thin round for detail, "filbert"(round and flat) good for edges and blending. Remember you can wipe off mistakes; the underlayer is dry and won't come off.
Don't make the whites of the eyes too white or they'll look like they're staring. Just make sure it's lighter than eyelid (use the same base but add some white, then blue if it's too pink). Same for mouth/teeth - but dark in corners (violet, red green) to define- sculpt the face with the paint. And when you're smudging don't forget what finger you've used for each color of paint.
As it turns out you also paint over pretty much all of the dark you put down earlier. In my case, I needed to thin the cheek and nose line and reduce the harshness of they eyebrow; for things I wanted to stay dark, I did this with a reddish purply mixture that nicely lightened the color; for things where I didn't want there to still be a line, I just painted over it with one of the skin tone, and the fact the paint was thicker this time pretty much took care of erasing the dark line.