1 bottle cap, 2 if you want a practice run. Not a bad idea to practice once if the cap you want to use is hard to get, or has any sentimental value.
2 small metal rings: I used small snap washers, which can be found in just about any craft or hardware store for less than a dollar for a pack of 1 hundred. Jewelry ‘finding’ rings would work just as well I’m sure.
One pair of pliers
One pair of cutters, something with a blunt nose or fine tip is best
A thin piece of wood, sturdy cardboard, or plastic to bear the brunt of the damage from the pliers. This layer can be anything so long as its about the size of your bottle cap, thin and sturdy enough to act as a scratch barrier and keep your bottle cap from losing its markings.
Estimated Cost: Less than $1… unless you count the beverage the cap came from… personally I was going to drink it anyway.