A quarter will yield a small ring, size ~7 or less. You can get a larger size but the ring will be very skinny. If you want a larger size ring, a Kennedy half dollar will work well. Again, anything 1964 and earlier will be silver.
Optional but recommended
Step 1: Find a silver quarter
DO NOT buy an expensive silver coin for this, you will be destroying it.
Step 2: Start tapping
**Do your tapping when no one is around, the tapping can get very annoying to others in the nearby vicinity.**
The more you tap the faster the ring will form. I was a casual tapper, I tapped while I watched TV. My first ring took a year to finish. If you're the impatient type you can speed up the process by using a hammer and a hard surface. If you do this you risk bending or warping the coin and your finished ring will look crude and rough. The small taps of the spoon give a much smoother beveled look.