I will detail two methods you can use to achieve these effects.
Step 1: Gather Materials
As many $1 halloween skulls as you wish from the dollar store or equivalent where you live
1 utility knife or exacto
1 can rubberized undercoating spray (method #1)
1 can of turpentine
1 can of dark wood stain (method #2)
1 can of grey primer
acrylic or water based airbrush paint
fine tip brushes
airbrush (you will of course need an air compressor if u use this)
Step 2: Cut away black plastic area between the jaws
Be very careful, the blade can easily slip and seriously cut you.
Alternatively you can cut the lower jaw away completely so that it sits flat as I did in image 4. Those are completed skulls.
At this point you should also take the time to sand down the lines formed where the molds connected during the manufacturing process
Step 3: Method #1
First take the can of rubberized undercoating and spray or brush a light/medium coat onto the skull and allow it to dry.
Next take the turpentine and soak a little into a spare towel. Gently rub the skull until the rubber has thinned leaving a thin yellowish coating.
Now grab a clean dry towel and very gently rub the skull. This removes rubber in the most elevated areas creating highlights.
Repeat this process until your happy with the highlights, shadows, and overall appeance.