A picture does it best... locate the two prominent, natural holes that lead into the skull cavity just behind the eye socket (optic nerve pathway?)... 1/16" wire looped through both and clamped.

Been a while since I looked at a deer skull, but if you can't use a natural hole, drill one. Antelope have the same optic nerve channel in the skull however.

The hanging point on the wall needs to be fairly high in order for the mount to hang flat, especially on a lighter deer-sized skull...

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Finished mount hangs nice and flat like this:

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“Perfection is Achieved Not When There Is Nothing More to Add, But When There Is Nothing Left to Take Away” Antoine de Saint-Exupery