Expostion: we learn that a series of concussions has caused a world-wide blackout, that the son was born after the blackout, that the boy's mother committed suicide soon after, and that the father, who is sick and has a gun with two bullets, wants to travel southwest along the road to the ocean.  The novel starts in medias res, so most of this expostion comes in bits and pieces of flashbacks strewn throughout the novel.

Rising Action/Complication: we learn that cannibals and bands marauders threaten the tandem, that the father and son must hide from them in order to survive, that the father and son must compete with these subhumans for food, shelter, warmth, and recources.

Climax: a cannible captures the boy and threatens to kill him.  The father shoots him, and they escape.  The man only has one bullet left for the two of them.  He ponders shooting the boy, should they get captured.

Falling Action: the father and son find a shelter that is stocked full of ample supplies.  This seems like a haven, a home, but the father knows that it is too exposed to be safe for long. They must abandon it.

Resolution: the father and son reach the ocean.  The father threatens to kill a man along the road, but the boy talks him out of it.  The father soon dies, and the boy walks onto the road.

Denoument: The boy is discovered by the man with the shotgun who has been following them for some time.  The boy goes with this man and becomes part of his family.

The first thing that will help is a solid understanding of the events that occurred in the book itself, and an idea of the order and timeline in which those events occurred.  Enotes has great resources for piecing together plots and timelines; I provided links below to a summary of the novel that will help to hone in on the most important aspects of the plot to use in your diagram.  Sometimes it is hard to know what to put in and what to leave out, especially in this novel, as all of it is so much the same--the father and son walking, walking.  So, the summary can help you to narrow it down to the main, and most significant events in the novel itself to focus on.

Secondly, to do a diagram, you need to figure out what events fall into what category of the plot structure.  There is the exposition, which is basically the set-up of the story, or the part where we learn the setting, characters, and the basic situation that they are in.  So, the opening of the novel, where the father and son are sleeping in the wasteland, and the first few pages where it is explained that they are in survival mode during a nuclear winter, that is the exposition.  Next, you need to pick several events that might lead up to the climax (which also means you need to decide what the climax is--in this novel, I would probably say that it is all leading up to when they reach the ocean; after the ocean, it is falling action).  So, list key events leading up to that climax--the barbarians, the food shelter, the survivors that they run across, etc.  Then, the climax is probably the ocean; after this, list the events that can be labeled the falling action (not many; their decision to go elsewhere, the stealing of their goods and the father's hunt of the guy), and then the resolution, which would be the father's death and the boy's acceptance into the family.

Organize all of these events into a triangle shape; for examples of this, just type "plot diagram" into google and look at the pictures of a plot diagram that come up, and that will help you.  I hope that those thoughts helped; good luck!