This must be put at the end of every sentence a Prinny speaks.
However, this rule has become fairly optional, since Prinny Kurtis and Big Sis Prinny don't use "dood" in their dialogue at all. Some Prinnies have trained themselves to use it less often, though they are ridiculed. There are various other rules when using the word "dood", such as:
It is not a real noun; people are not "doods". For example, "Master Dood!" for "Master Etna!"
It cannot be used in puns. For example, "Just doing my dood-y, dood! Tee-hee!"
Eyaah! So this is the murder scene, dood?! Why the hell was that still here, dood?! Dood, what's going on, dood?! Dood!