Since no one has chimed in yet, maybe I can help.
First and foremost, Google is your friend when looking stuff up. Now, that being said, using acrylics as spraypaint:
If I'm spraying with acrylic, I usually use it in conjunction with
compressor and
spray gun kit. You'd also need a
hose to complete this connection from the compressor to the spray gun. That can cover large volumes.
This video should help get you started with acrylic as a spray paint (I apologize for the obnoxious songs in the videos I link to).
For the most part, I like to use spray cans whenever I can and use the acrylics for accent
airbrushing . I use a
much smaller gun for the airbrushing and adjust the pressure coming out of the compressor to around 25 to 40 lbs. depending on how the paint is reacting.
Here is a skull being done using airbrushing.
I can't help you much with the design of the skull other than a few ideas. You could:
1) Start with
full sheets of adhesive paper, print out your design on a printer, and then exacto knife the picture out of the paper and adhere it to the helmet and then start painting.
2) Use the same adhesive paper but remove the top sticky white sheet, keep the bottom nonstick sheet, and place down strips of blue tape to cover the entire 8.5" x 11" page again. Free hand the skull on the blue tape. Cut out the design you want and place the blue tape on the helmet and paint.
3) (Here is a real time intensive solution) Use really
small blue tape to free hand out what you don't want painted on the helmet. It's great for curves and what not. See
this guy's video to get the gist of it.
4) This is what I'd do, Use the airbrush technique from
the video above.
I hope that helps at least answer a few questions.