Take time to get a good composition on the page.
Make sure yoou have the eyelash line, nostrils and mid-line of the mouth drawn in and try to keep them in the right place as you work.
As you progress, you can check your measurements buy using 1 feature as a control measurement. eg, How many eye widths is the length and width of the nose, width of the face or between the eyes. Take lines up and down to see what is in line with what.
When drawing wrinkles, form these with what is around them - you can leave some out.
There can be blue, mauve, green or yellow hues in skin tones. Build the tones up in patches.
Use colours in shadows simetimes, eg red or blue
Try not to oversmudge - towards the finish you can leave pastel on the paper surface.
Highlights aren't always white, they can be the lightest hue of a colour
Use a darker background against the lightest side of the head and a lighter background against the darkest side.