A plot skeleton or diagram is the most basic part of your story. Each part gives you an outline of what your story should be, or a guide to follow, which can then be filled in with the muscle and tissue of your work.
What is exposition? This is the beginning, the setting, or the relative background that the reader needs to start the story. At the very, very basic, it is where your story will start.
Example: We meet Jane. Jane lives in New York City. Jane has been there for a year, and she is searching for a better job.
2. Inciting Incident or Conflict.
This is where we find out what the problem is, or what sets off the problem. Every story has a problem; without one, there's no story to be told.
Example: Jane goes to a job interview, where she learns that she can't be hired because of her degree.
3. Rising Action.
This is where your characters will deal with the conflict/problem. They will go through complex emotions, and this is where you can spend a lot of time showing your reader who your characters are. How do they deal with the problem? Are they likeable? This is your moment to really flesh out who your character is.
Example: Jane has to decide how she is going to get around not getting her dream job. Does she pout? Does she panic? Does she pull herself up by her bootstraps and go back to school? Does she give up?
This is one of the most important parts of the story. This is where things come to a head, and often where the character(s) have to make a tough choice.
Example: Jane can't go back to school because she has no money. Her only option is to work her dead-end job to make ends meet. She realizes that her life isn't going to be what she thought it would.
5. Falling Action.
Your character(s) have come to terms with what is happening. They've conquered the climax and the struggle, or at least have accepted whatever the problem is. This is where catharsis is very important to a story. A story doesn't have to have a happy ending as long as it fulfills the reader's need for resolution.
Example: Jane stays at the job she hates and tries to make the best of it.
This is the ending. The Resolution and the Falling Action are relatively short parts of your story, and often they can be meshed together in a way. They both help to accomplish the same thing, which is to wrap up the story and leave the reader with a satisfying ending.
Example: Jane finds that she's happy now that she's accepted her role.
If you're unsure about where things belong on your diagram, start with the easy stuff, mainly the 6 things listed here. You can make your diagram as simple or as complex as you'd like. Remember to try not to force too many things into your rising action or climax. If you find yourself having lots of small climaxes, try to break them up into their own diagrams. This is often the indication that you're going to need to write multiple stories, not just one!
What do your diagrams look like? What are areas that you struggle with?