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.
Step 3: Size your ring
This site will convert your ring size to inches or mm.
Tap the ring until you ring size is pretty close. You can be off by a 1-2 mm because there will be extra space in the center to play with.
When you reach about size 6 you can see the date and the words "Liberty", "Quarter Dollar", and "United States Of America" on the inside of the ring. This will be your only proof that you made this from a quarter.
Step 4: Make a hole
**Also note that drilling the ring gets it very hot. It can burn you, handle with care**
I drilled a hole in the center followed by a bigger hole and so on. Drill out as much of the center as you can. A dremel will probably work well here. I didn't have one for my first ring. Once you have enough of the center out you can grind away the edges with a file or dremel.
At this point you can play around with the size. Try the ring on and grind away until it fits right.
Step 5: Polish your new ring
After that you are done.