There are many ways to affix pin backs to polymer clay projects. A simple way is to affix the pin back to the back of the clay item and then affix a little patch of clay over it with liquid clay and bake.
By far the neatest way is to embed the pin with just the pin and hook showing. To do this, cut a piece of clay the same shape as your clay bead/broach. Cut 2 small holes or + shapes to allow only the pin and catch ends to poke through. The backing piece is then affixed to the back of your bead / broach using liquid clay. Trim edges and bake.