First of all for good results you need to have some images, some references. Better 2 or more to better understanding of shapes of your object.
For sketch i use a hard brush.
For this image I use just custom brushes with defferent value of softness and transparency.

I do not like to start with black and white drawing, but in this case I had a little time.
For the background and initial image I use a soft brush (70% -90%) opacity 50-70%

We need to get the correct form, shape, correct light and shadow in the beginning. And then just to improve it.

When the main picture is wholly satisfactory, I reduce the opacity and brush pressure (20% -40%), and try to improve the details.

I never try to make my image very smooht. Especially if it's skin. Otherwise it will look too rubbery and unreal.

Now color.
This part just for beginners, of course ;)

Edd new layer with blending mode color. And fill it with color which you think you need for your image.

Not too bad.
But for figure did not look too boring, I put some additional color on the same layer of semi-transparent soft brush. You can apply a little warm colors in light areas (for example, pale yellow or greenish) and cool colors in the shadows (blue or violet).
Do this carefully so that image has not turned out too many-colored.

For iris I use I use soft brush at the beginning and maybe a litte blur tool. And after that hard brush for iris details.

For eyelashes I use hard brush (with turned on pen pressure).
Sometimes I blurring its tips. Also finger tool can be useful.
The same for eyebrows. I bluring some hatching and some make again in hard.

WOW Forgot about the texture.
In the end make a little texture on skin.
You can use a bit of noise in certain areas or special texture brushes. Or you can even make them yourself. But don't use texture all over the skin.

The end.
LOL! It turns out that to make the description is terribly difficult ;) Harder than to paint.
I hope you enjoy it ;)

at the request of the moderators add a description of a brush
1) easy way
take brush soft 75% spasing 270% and following settings opacity 50%

result

2-d way
make some spots with noise settings. darken inside and use finger tool to make shape a little less round. Use image for making own brush with spasing about 25%

the result of these ways is very similar

and finally the the references.
I took this 2 image and created something new from them