This is a variation on an old Norse technique found in Egil's saga. Originally a horse head was strapped to the top of a pole and faced at the direction of an enemies land. Freya was a Norse manifestation of the PIE concept of a Horse goddess, or a goddess connected to the land, who represented sexual freedom, and sovriegnity. Rhiannon, Macha, and Epona are Celtic horse goddesses. The opposite of a horse goddess is the cow goddess, a goddess who is connected to house home and hearth, and represents wifeship. The Nithling pole works by profaning Freya and thus angering and terrifying the land wights so that they attack and destroy your enemy and his land.
Most people won't want to go get a horse's head these days so typically nithling poles are done by using runes. Take a pole and in the top of it rynst runes for your enemy's destruction. It would also be appropriate to carve a horse head on the top of the staff so that it can be used to direct the force of the nithling pole. Stake the pole at the edge of your property and face it in the direction of your enemy. You might hang things off of it like horse hair, chicken bones, and raven feathers to make it more terrifying.