They say when you use a stencil, it should not look like you used one when your done.
With that said, give your skull some more detail and shading.
Work the teeth in. Give him some personality and attitude.
Are you using any skull reference pictures, if not you should.
It will help you to know where to put the shading and details.
Your doing good, just practice, practice, practice doing the basics.
Dots, Lines, and of course the Dagger Strokes.
What kind of paint are you using ?
To make things easier for yourself, you should try some ready airbrush paint like, Golden Airbrush Colors.
As a beginner, you already have enough on your plate. This way you won't have to worry about thinning out paint just yet.
Have fun and spray away
It does look good. Gioart is right. Play with the thickness of your paint and use a quality paint if you can. The added money, very little in most cases, is well worth it in the long run. Craigs list is full of airbrushes being sold by people who got frustrated and gave up (I came real close) LOL, because of frustration. One of the best rewards of practice is to be able to adapt as you go and make corrections on the fly.
Don't be so dramatic! That sounds very cold to us Yanks. Tell deadeye it's about 40 degrees F. It's cool here too. It got all the way down to 42 F and is only 71 F at the moment. Of course, we do have the sun also.