I also have this project as a tutorial on my blog.
Step 1: The Stuff
Polymer clay - two colors; one for the background, one for the inlay
Pasta roller or clay roller
Dry paint brush
Exacto knife and/or long flexible blade
*Not all rubber stamps are going to give a good inlay image and, the inlay design may not be as detailed as the stamped image. Test your image on some scrap clay and look carefully at the shapes in the smooth top surface of your clay as that will be the design of the inlaynot the deeply set detailed design that normally appears when it is used as a stamp.
Step 2: Preparing the Design
Dust the rubber stamp with corn starch and brush away the excess. (I use corn starch because it dissolves in water and washes away easily.)
Step 3: Stamping the Clay
Bake the clay according to package instructions. When the clay is cool, rinse it off with cold water to clean off the corn starch. Make sure the clay is very dry before moving on.
Step 4: Creating the Inlay
Remove as much clay from the top as possible. Use your clay cutter to scrape off excess clay. Any clay not removed here will be manually sanded away. (I have had a modest amount of success removing excess clay by running the piece through the pasta maker again. I have not had enough success to fully recommend it.)
Step 5: Finishing
Once it is cool, use waterproof sandpaper and soapy water to sand away any excess clay from the top. Pay attention to the design, rinsing as you go and only sanding where it is necessary.
It is possible to use progressively finer grades of sandpaper to create a glossy finish. However, I prefer to glaze over the top when the piece is finished.