On these magnets I'm using stamps to press into the clay. You don't want to put to much pressure when pushing the stamp because you will get the side of the rubber on the clay. Other magnets I do I mold the clay into shape, flowers, what ever my mind comes up with.
I use another size cylinder to cut out the stamp imprint. I use these to get a great circle. I use a knife to cut around ones that I want to be odd shaped. I will use the metal ruler on it's edge to make stright line cuts.