Quite a simple one. Once you've completed the intro, butchers will be available around the map as indicated by a cleaver icon. As you ride around, shoot and kill animals and be sure to skin them. Once you have 20 animal items (hides, meat, feathers, horns etc) travel to the butchers and sell them.
They don't have to be 20 unique items, mine popped selling 3 raven flight feathers. However, as has been pointed out, you need to sell them in a single session so as not to reset your counter.
As Notorious Gronk points out, try killing turkeys as you get 2 meat, 2 feathers and their carcass from each kill.