Step 9: Sanding
I use wet/dry sandpaper, start from the coarser grit to get rid of all the remaining raised parts of the impression. Make sure the clay is perfectly flat before you proceed. Then build your way up to remove all the scratches and make the clay smooth. Don't sand the clay dry, you risk breathing the dust and I'm quite sure it's not healthy.
This is the list of grits I use:
320 - Coarser. After sanding the polymer clay will look opaque and the mica shift effect will be less noticeable. Don't despair, that's normal.
1000 - You can stop here if you want, and the clay will look fine.
2000 - Softer. Going all the way to 2000 makes the clay really smooth and shiny. I love touching clay that is sanded with a 2000 grit, it's very smooth and soft.