Bottle Caps - Gatorade and Milk Caps are the best because of the larger size.
Foam Letters or Shapes - This is where it can get a bit tricky. I bought foam letter stickers (oops), when you stamp the letters will be backwards. I realized this just as Steel and I were getting ready to stick the letters on the bottle caps. SO - I highly suggest that you either stick to the foam shape stickers or letter without the sticker so that you can adhere them backwards so that they will stamp properly.
Glue - For the non-sticker letters!
Ink Pads - I was pleasantly surprised that these have come down in price over the years, I found them at the craft store for a buck a piece!
The How To
1. Stick shapes to the bottle caps. (See below how I improvised using letters because I bought the wrong ones!)
1. Apply foam stickers to bottle caps.
2. Stamp from right to left.
3. Reveal your secret code in a mirror...