Hello bennie! First off welcome to the Orange forum!! Secondly, the Power Series is excellent training!! Let it soak in and it will definitely help. I always preach on 3 things, and if you can get them dialed in your airbrushing experience will be a more enjoyable one!! 1. TRIGGER CONTROL 2. REDUCTION 3. AIR PRESSURE Once you dial these three things in, things will start falling into place a lot quicker for ya!
As far as critiquing your skull, I would definitely recommend a reference photo, especially just starting out so you can really see where the shadows and highlights are, that's very important in rendering a piece. If the shadows are in the wrong place or the highlights, it will make the piece lose all its perspective!
On your skull, what I mentioned above falls into play, there's no hard evidence of where your light source is coming in from. When you start a piece, decide during the sketch phase where your light source is going to be so you can add the appropriate details to make the piece look 3 dimensional.
When you use a stencil, just lightly spray over it, very lightly, just enough so that you can see the outline, kind of a map of where everything is. Then freehand the rest with soft gradients, using a freehand shield occasionally to keep some of the edges sharp.
You've already conquered a feat that many are afraid of, you posted your work and asked for advice. This will help you more than anything and many are afraid of what others will think of their work, but we are all here to help one another, that's what these forums are all about.
So I think the main thing I would say you need in the pic is a light source so you can start throwing some shadows and highlights on it. Just for an example, around the upper teeth, there would be shadows around the tops of them, under the nose would be darker with a shadow unless the light was coming from the bottom front. Lil' things like that.
Good luck my friend!!