A few tricks from a fellow beginner.. pretty much how I'm doing it now after a few attempts at it. Most of what I do I've just picked up from other tutorials and vids. All hand brushed, but I'm sure an airbrush would make it easier.
Invest in a good daylight LED light for your desk, and some magnifying devices.
I use a desk like with magnifier like this which is awesome:
https://www.stonesfinds.com/wp-content/uploads/Artisan-Lighted-Magnifying-Daylight-Desk-Lamp-with-Clamp.jpg
And I use this for getting really close with eyes and fine features:
https://www.jaycar.com.au/medias/sys_master/images/h9f/h39/8815176253470/QM3511ImageMain-515Wx515H.jpg
Start by painting a darker version of your final flesh tone all over whatever primer you're using.
For an idea of what colours to use for various skin tones, there's a great reference online - look here at the file called "flesh shades":
https://drive.google.com/drive/folders/0BxBp_BdjWzPKLU00Z1MyazZkR3M
Start with one of the darker shades as the base (think deep tan) and then move up to the lighter shade. I generally mix in the lighter colours into the darker base as I work up. Each coat is very thin to avoid clogging the pores and lines in the cast. I use distilled water with flow enhancer to keep the coats thin but avoid them pooling into beads too much.
Flow enhancer looks like this:
https://www.artsupplies.co.uk/img_alt/Flow-Aid.jpg
Also, you can use the thinners for your particular brand of paint. Make sure you're using the correct thinners for your type of paint too, there are lacquer and acrylic thinners in the same brand, I've made that mistake before..
When you're getting close to the shade of skin that you like, I usually flick some watered down reddy/brown paint onto the face to give the skin some mottling.
Then you follow up with your lighter skin shades which tones down the spots and adds some depth.
You get something like this:
Follow up with some lighter flesh colour to bring the skin closer to the shade you want. It's hard to judge the flesh colour and whether it looks accurate, so when you think you're close you can do a thin coat on the hair, lips, eyebrows, and I usually paint the whites on the eyes too just as a first coat. Eyes are a mix of ivory, a tan skin colour, and some grey for me. You don't want stark white.
Once you've blocked the colours, you can see better what the skin tone is looking like and whether you're happy. It will look something like this:
Refine the skin a little more. I mix up a watery red and start shading the reds around the eyes, cheeks, forehead and nose. This is where it starts looking more human. From here on out, it's just more thin layers of reds, skintone, bring out the eyebrows and so on. Keep the paint thin, wipe off excess on your brush with paper towel and on your own hand before touching brush to sculpt so that you don't end up with a watery run. You need colour on the brush, but you don't need lots of it because you will build it up.
Keep going, making very small changes. I dry each coat with a hairdryer usually, or place it infront of a small heater. Otherwise if you do the next coat too quickly, you can easily wipe off the paint underneath or lift some of it up and create a nasty mess.
If the underlying coat is dry enough, if you do make a mistake or go too thick with what you are applying you can rinse off the brush and use the wet brush to wipe away your mistake. Then dry the brush on some paper towel and soak up the water on the sculpt before it dries again. This works very well.
Eyes are a PITA. I still can't do them well yet. Use a small brush + head mounted magnifying glass to help here. Use Dullcote to seal your work before trying the iris so that you can wet your brush and wipe off mistakes without messing up anything underneath.
Dullcote will also make your paint look nice and dull instead of shiny.
Once your eyes are done, put on another coat of Dullcote. Let it dry well, then paint some gloss on the eyes and a little tiny bit on the lips.
Paint on your dirt / blood etc last if that's what you're doing, after everything else is done.
And here's a Jack Bauer I did last night following the same steps. The lighting isn't great but in the non-bloody picture you can kind of see the red shading + mottling I'm talking about. Going from your basic skin tone to a more realistic version of the face really seems to just be about adding red shading where it's needed to flush and blemish the skin. I always see a major improvement and feel a lot better about the sculpt once I start to add some red shading in the areas that need it.
