I am both a tile installer and medallion manufacturer. The best way is to lay the field tiles out underneath where the medallion will rest. Next, place the medallion on top and draw around using a permanent marker. Next, create a numbering system, marking the back of the tile as well as the floor where the numbered piece goes. To make the cuts on a standard wet saw simply make a series of parallel cross cuts about a half inch apart, stopping at the cut line. Then gently break away the cut area and clean up the radius with tile snips and then a grinding stone. You may want to try a few practice cuts before you start. Good luck!
Another alternative is to use a tile bandsaw which will allow you to follow the curve on your cuts, that is if you can find one to rent. The one I use is the Diamond Laser 5000.
