A shofar is made by hollowing and heat shaping the horn of a ram or another kosher animal, never reinforced, painted over its natural surface, or built up with additional material the way a musical instrument might be, since Jewish law requires it to remain recognizably a natural horn.
What is specifically excluded
A cow or ox horn is traditionally avoided regardless of how suitable it might otherwise be, out of reluctance to use an animal associated with the incident of the golden calf, a restriction that narrows the acceptable source animals more than physical requirements alone would. Beyond that, the horn is not decorated in ways that would obscure its natural form, though exterior engraving that does not alter the shape is accepted by many authorities.
Training, and what happens to a cracked one
Blowing a shofar correctly, producing the required sequence of blasts, is a skill practiced and often formally assigned to one trained congregant, a baal tokea, rather than left to whoever is available. A shofar that cracks or otherwise stops producing a proper sound is not a text bearing object, so it carries no burial requirement, it is simply retired or, if the damage is minor, repaired by a specialist familiar with the horn's natural structure.