Dry mix oatmeal, salt, pepper, sage, onion, and garlic.
Knead in the vegetable shortening, then knead in half of the butter. If consistency is not sticking together to form a clump when kneading, then continue to add butter in 1 tbs. increments. Increase amount of butter if necessary.
Fluff mixture so it isn't hard-packed, and then stuff bird with mixture.
On edit: I generally cook turkey slower and longer than most and add water to the pan for gravy use. I baste it several times throughout and shoot the water with a squeeze bulb type baster directly into the bird's cavity to keep the dressing moist.