Create a setting. The setting establishes the tone for your play or movie and gives the characters space in which to interact.
Make a list of main characters, and make two columns of attributes next to each. One column is for physical attributes like height, weight, build, hair and eye color. The next column is for personality attributes. Finally, select at least one character---but not all of them---to have a strange quirk, and write that down as well.
Jot down in a few words what the main conflict in the story is going to be. It could be a crime, an illicit love, a disagreement or even internal conflict.
Note how the conflict will be entered---how the audience will learn about it---and how the conflict will be resolved. Conflict and its resolution are your main plot points.
Return to your list of main characters. Decide how each character will be introduced and what will happen to him or her during the course of the play or movie. Make a note of any supporting characters who might be needed as well, in addition to their general characteristics.
Begin writing. Select a place from which to enter your plot, which characters to begin with and go from there. Make sure to follow the plot points you laid out, and refer frequently to the list of characteristics and quirks for main characters, as well as when and how they should be introduced or removed from the action.