jhill it's all on the computer
i create the sign face in illustrator then export it as a .dwg
then open it in google sketchup (
http://sketchup.google.com/) it's an open source program
create my structure add the textures
then export from google sketchup in 3 different exports (1.shadows only, 2.full color display with no edges displayed, 3.export with edges shown ony)
import into photoshop, layer the images with the shadows on top, color in the middle, and the edges on bottom
put a mask on the color layer and use a brush to erase away
that's the basic idea there are a few more technical steps, i wish i were good enough to do it in water colors though