Luck of the moose's guide is very thorough, but i have some things to add/ change.
You MUST be BEHIND the enemy to "assassinate" them (or on top of them if they are prone, basically stab them in the back). think halo reach if you've played it. a slash from the knife from the front is a 2- hit kill i think (although you are usually too dead to swing again).
I dont think you have to hold the button down(i may be wrong, but i have a few dogtags now and didnt consciously hold it down...), and you do not have to first equip then knife the press RT, all you need to do is go behind them and press RB, even if your gun is equipped.
otherwise moose's tips are very helpful!