I have made drop target decals for my Black Knight and Millionaire. I print the art on single sheet labels and then laminate them with a self adhesive lamination sheet (all available at Wal-Mart or Staples etc...). When I apply them to the target, I also apply a thin layer of spray adhesive to help keep them secured (also available most places).
I actually found a tutorial online somewhere that gave a step by step on the above process. I can't remember where it was though. I haven't used it in a while. The process is very simple, so I don't think you'll need the step-by-step.
The biggest issue is getting the art correct. If you can find a downloadable image, that's best. If you can't, you have to scan an original and clean it up in a image editing software package (had to do that with Black Knight).
Below are the images I used for BK and Millionaire.
BK.PNG Ma.PNG