Leather must be moistened by rubbing a damp sponge on it. Place leather on top of a firm surface like marble or rubber board. Use a wood or raw hide mallet head to strike the top of the stamp to obtain its impression in the leather. Control over degree and force of striking depend on the case.
This stamp may have a surface like this picture but they are not cracks. They come from the shape of the mold and it is not a sign of defective product.