Identify the major characters, plot events and the setting. Whatever length your synopsis is, you will need to include these details.
Write a one-sentence description of your play. Even a long narrative should begin with this, because it gives the reader vital information immediately. A one-sentence description of "Hamlet," for example, would read: "Hamlet, the prince of Denmark, seeks revenge on his uncle Claudius, who usurped the throne, murdered Hamlet's father and married his mother."
Write a description of the first two scenes. Write it like you are telling the plot to a friend, rather than trying to touch upon every small detail. A synopsis for the beginning of "Hamlet" might read: "Two guards and a scholar named Horatio see the ghost of Hamlet's father, so they bring Hamlet to see it. The ghost tells Hamlet about his murder, and that Hamlet's Uncle Claudius committed it." This touches upon the major plot points early in the play without getting too bogged down in detail.
Summarize scenes or groups of scenes through the rest of the play. Pay close attention to including the most crucial plot points in your narrative.
Include the ending: the point of a synopsis is to summarize the entire action of a play, so don't hold back the ending for dramatic effect.
Reread your synopsis; check for typos and misspellings, but also make sure you did not forget to include any vital scenes, characters or details.