Make the brownies according to the instructions on the back of the box, or your trusty made-from-scratch brownie recipe.
Bake, and allow to cool completely.
Once the brownies have cooled to room temperature, cut them into triangles. How large you make them is totally up to you. Just make sure you have plenty of room to add the antlers, eyes and nose.
If you choose to ice your brownies, simply stick on the antlers, eyes and nose. If your brownies are not iced, use a tiny dab of cookie icing as glue and add the antlers, eyes and nose.