Geoff Williams, in his book CNC Robotics took the following approach:
1. Plot the image on the surface with a permanent marker that resists etching paste, He found that the Pilot Metalic Marker (extra fine) would work.
2. Then added a border around the plot with a wider version to prevent "spill over".
3. Let the ink completely dry.
4. Read the safety warnings on the paste.
5. He applied the paste with a small piece of wood (bet the caustic stuff eats up the metal bits in brushes)
6. He let it dry for 15 minutes (plus or minus based on brand of etching paste).
7. Wash off the paste and then clean off the ink with lacquer thinner.
There's been a lot of plus/delta said 'bout his book. But this looks like it might work!
You might find all the bits to do the same thing at a crafts shop (Michaels or JoAnns - chain crafdt stores) or a stained glass shop.
Jim
PS - if you want to buy the book - best bet I found was thru overstock.com