I thought I would share this
I have experimented with several methods, but so far this has been the easyist and the most effective.
Firstly you get a large sheet of that duraseal stuff (you know the sticky plastic that you cover books with).
Now get a a4 sheet of paper and use that as a template to cut a a4 sheet out of your duracell.
Now pick your design on the computer, not to complex and preferably a vector or single coloured image. Put the duracell in the printer so when it prints It will be printing on the paper backing. Print it.
Cut it out, when ready to paint peel off the backing stick it on and it masks like a profesional.
OUTCOME: