Step 6: Ta-DA!
If you were to allow the etch process to continue, it could cut all the way through the metal, creating a cut-out design.
You could stop here, or you could get fancy. I got fancy.
Using another idea, I opened the tin, and wrote my message on the inside of the lid using a sharpie. Any non-water soluble pen should work, but you will need to get it off again, so don't go crazy.
I covered the top of the lid in tape again to protect it. I then etched the lid portion using the same process. The areas where the sharpie was resisted the etching (pretty well anyhow) leaving the writing behind. After it etched a bit, the cat knocked the whole damn thing over. It had worked pretty well by that point though. I rinsed it, removed the tape from the top and lightly used a green scrubby to remove the sharpie revealing the un-etched area underneath.
I took some spare fabric we had laying around and glued it to the inside. I would have preferred felt, but you work with what you have. I'm filling it with chocolates for my beautiful and fabulous girlfriend. Sorry about the bowl and scrubby dear.
Good luck, be safe and have fun!