1. A scene-setting preposition, usually just one word will do. Classically, this has taken the form of “When”.
2. The first free-form part of The Song, this should further develop a sense of place, time, urgency, activity or any other useful stage on-which Diarrhea could plausibly occur. The only true requirement for this element is that it rhyme with element 4, below.
3. Insert the mandatory conjunction AND. There are very few hard rules in creation of The Diarrhea Song, the use of and in the middle is a tradition, and as such, an exception to the rule. Please use it.
4. The second free-form part of The Song, this is the punch line. It is here that it will be revealed how one knows that it is, in fact Diarrhea, and not something else. In other words, describe the result of the Diarrhea (i.e.. “You feel something burst”). Keep in mind that this should rhyme with element #2.
5. Add the famous ending that truly identifies your work as part of the long legacy of The Diarrhea Song: "Diarrhea, Diarrhea". If you are a traditionalist, you may prefer the more automata-poetic, "Diarrhea, Plop, Plop, Diarrhea". Both variations are correct, according to the linguistics department.
e.g.:
When you think your friends are joking
but your pants are brown and soaking:
Diarrhea, diarrhea.
When you're standing in a shop
and you hear a sudden plop