start with spots and squiggles snag.gy/ctuep.jpg

light yellow in the air brush and set the Mode to Luminosity snag.gy/W4CtB.jpg

More magical stuffs snag.gy/5h0bU.jpg using what ever you want lol  

make a layer above it, check the clipping group box and add what ever colors you want your magic to be using air brush. 

hope that helps