Would Obama just call my boy Kim Jong, and say, ey b0ss I'm gon' be fuckin wit you today?
Is there a set way?
Or is there suddenly an army at your door and you get to respond?
(sorry about the profanity. And I have no idea if this is the right place for this.)
Usually the ambassador of the country delivers a letter to the foreign mister of the country being attacked (assuming there is a diplomatic relationship). If there isn't, the declaration of may be done through a third party.