There are several methods of stamping into oven bake clay. The first, and probably my favourite, is to take a clean, dry stamp, roll out a flat piece of clay (not too thick - remember, you want to wear this as jewelery and it will bake hard so you don't need much more than an eighth of an inch) and simply press the stamp into the clay to leave an impression. If you don't like the first try, roll the clay up into a ball and start again! Cut around the image with scissors or a craft knife, smooth the edges with your fingers, poke a hole with a cocktail stick or similar (if you need one to hang the item because you are making earrings or a pendant) and bake. This can look very sophisticated, especially if you use a metallic clay such as Sculpey Bronze.
When you bake your item, you can leave it flat, or curve it, curl the edges, etc. - which can look very effective for a stamping of a leaf on autumnal colours, for example.