Fill the balloon with water . This image illustrates why you don't completely fill the balloon you should always leave some room in the neck of the balloon, then as the form dries the pressure has somewhere to go and your clay wont split like that.
Ideas for home-made stamps. One method, aside from hand-carving using other objects, is to press other objects, such as buttons, into the end of the clay handle. Handles are simply cylinders, which can be extruded if you have an extruder.