Kill things and drag the corpses to a Carcharodontosaurus. Clear every wild carnivore nearby first, Argentavis above all, because anything that eats your offering first wastes it. You will know it accepted when it rears up, sniffs the air and walks over. Once its trust meter is full you can mount it, and from there the tame runs on Bloodrage, the buff it stacks by killing while you ride, so keep hunting. Leave it fifteen minutes and the stacks rot, and every stack you lose costs effectiveness as well as progress.