The best advice I can give you is make a practice cake with white icing and "hAVE AT HER" That's how I learned to use mine!! There are several books out one by Winbeckler on his site www.winbeckler.com.Just practice,practice,practice!!
These were helpful to me when I started...
http://www.sugarcraft.com/catalog/airbrush/lessons-AB.htm
http://www.sugarcraft.com/catalog/airbrush/abinfo.htm
This is an excellent resource also. Pages and pages of airbrushed cakes. Each cake comes with airbrush instructions so just pick an effect and follow the instructions for that cake.
http://www.pastrywiz.com/cakes/cakedecoration0003.htm
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