“Osteomancy” is the general term used for a broad collection of practices involving the creation of a talisman or fetish, either out of an animal’s remains or depicting an animal, and then using it as a source of power in the casting of spells or rituals.
Whatever method a mage uses to represent the animal — whether fashioning a talisman out of the bone or tooth of a beast, carving its image into a piece of wood, embroidering it into a piece of cloth, or anything else — they call upon the powers and properties of that animal to perform their magic. This is thought by scholars to be a means of tapping into the energies given off by the Primal Wilds, but the specifics of how it happens are still unclear, and thankfully there is no need for Osteomancers to bargain with the Primal Lords of that realm to access this power.
Osteomancers are notorious for finding many different ways of achieving the same effect, often calling on beasts outside of the “accepted” bestiary in ways that make sense to them, even if any other Osteomancer would be unable to use the same talisman and invocation to cast such a spell. This kind of variation happens particularly frequently between practitioners of different nations, as each ecosystem has its own quirks and useful beasts for Osteomancers to call upon.
If you wish to play an Osteomancer, craft or purchase one or more talismans representing the beasts you wish to call upon. Use them as the source of your character’s magic, roleplaying channeling power from them and into whatever spell or ritual you’re casting. Use verbal chants or invocations calling upon the beast(s) and its powers.