Well, it's the highest point on the neck.
Graphic material, warning (forensic photos + neck dissection showing blood vessels)
(Click the images to reveal the content)
Under the jaw, more or less at the height of the Hyoid Bone, which is above of the Adam's apple (laryngeal prominence). The ligature knot should be behind the neck in the center (it is always advisable to use a self-tightening knot such as the Arbor Knot, for example)