This will drive the coin down the mandrel getting it to fold over into shape.
You place the coin on top of the mandrel with the mandrel on a hard surface, cover the coin with the PVC and strike the top of the PVC with your mallet. Tip: some people use 3/4 inch cut piece of PVC pipe to assist in working the ring. Before you continue any further you should sand the punched/cut portion of the ring, this will help prevent splitting as you continue to work the ring. The result is shown on the brick, this is a much better starting point than a straight coin. I wrapped the ring in some paper towels, and using the parts that came with my doming set pressed the coin into the half circle steel form. Instead of starting with a flat coin, I choose to get a head start on the process a simpler way - using my press and a doming block (doming block $40). This is done by placing the flat coin over the tip of the Jeweler's ring mandrel (Amazon $9.95) and then working it down by striking it with a rubber/plastic mallet (Harbor Freight 2' Teardrop Mallet $9.95).
Once the coin is softened we have to start shaping it (soft is a relative term, do not expect that you can bend it with your hands).