A friend and I decided we'd like to make rings for fun over our winter break, so we did. Luckily, I had a forge sitting around that I had made in high school. I made it out of fire bricks and welded a steel frame to contain them. I also made a forge burner torch.
Starting off, we cut off a length of bolt about 2 inches long. This included the threads as well as the head to the bolt. After we cut it off with a grinding wheel, we heated it in the forge and hammered down the bolt into a round disk.
We then drilled a hole in the center of the disk and mounted it onto a drill. Mounting it on the drill gave us an easy way to make sure the ring was perfectly rounded on the belt grinder. After grinding them down, the inside diameter was filed to size. Once the rings fit, they were sanded and de-burred, and then they were completed.