Let's start, OK?
You need to have at least a 5 mm thick (minimum) disk bead. I used a 16 mm ring mandrel to make my disks. If you need help to make a ring/disk on a ring mandrel, please view my ring tutorial, also in this blog. (http://deesignedbeads.blogspot.com/2008/11/rinig-tutiorial.html)
I am going to cut a 1/4 wedge out of the ring. With my first attempt, I just cut a ring in half. It really did not look good. If you look at most hoop earrings with a stud post, you will see that they are about 3/4 of a circle. I drew a circle on paper and crossed it. This allowed me to mark the cutting lines correctly.
Since I use my normal diamond bit that I normally use to clean beads, it will eat about a mm of glass. For this reason I start cutting the ring a bit inside the 1/4 area that I want to remove from the ring. It will also allow me to clean up the ring after I completed it, and ensure that both rings is the same size.