Dave Barry is one of my all-time favorite humorists. Here’s his take on how to diagram a sentence.
Ask Mr. Language Person
Q. Please explain how to diagram a sentence.
A. First spread the sentence out on a clean, flat surface, such as an ironing board. Then, using a sharp pencil or X-Acto knife, locate the “predicate,” which indicates where the action has taken place and is usually located directly behind the gills. For example, in the sentence: “LaMont never would of bit a forest ranger,” the action probably took place in a forest. Thus your diagram would be shaped like a little tree with branches sticking out of it to indicate the locations of the various particles of speech, such as your gerunds, proverbs, adjutants, etc.
Thanks to The Passive Voice.