You can stamp after baking as well.
1) Create the item you want to stamp on.
2) Bake the piece, sand if you want to.
3) Rub the surface of the clay with rubbing alcohol to remove oils in the clay. DON'T TOUCH THE SURFACE TO BE STAMPED AFTER YOU DO THIS.
4) Stamp the clay.
5) Re-Bake the item for about 10-15 minutes.
6) While it's still hot, dip the item in Future Floor Polish (I got a huge bottle at Target for $5, and it's lasted me more than a year). Keep a rag on hand. If the ink runs, you can wipe off any areas that ran with a little bit of rag or cotton swab dipped in Future.
If that doesn't work, you can draw on the surface of the clay with Sharpie or Prismacolor marker and follow the same process. Prismacolors run like crazy when they're dipped, so it takes some practice.
were made with Sharpies and Prismacolors on polymer clay.