The nickels were hammered flat, then filed into the correct outlines. I then used paperclips to create the indented lines, and sanded/polished the pieces smooth. Small neodymium magnets were epoxied to the backs.
The heart was my pilot project, and thus the simplest. The yin-yang is a bit more complex - I sanded the entire piece with 100-grit paper, then taped over one half of the figure. I polished the exposed metal, then removed the tape hoping to have a contrast between the halves. I consider it a success, but it is quite subtle. You can see the effect best in the last picture where the heart and the yin-yang are juxtaposed on the fridge.