Decide on the structure of the play. A play can have from one to as many acts as you want. The most common narrative structure for theater is three acts, but this is not a requirement. The structure of the play will dictate how you organize the scenes of the play. Even when not using the three-act structure, basic storytelling remains the same. All stories should have a beginning, middle and end.
Describe the characters in detail. Characters are what the theater is all about. In movies, the characters can hide behind special effects but in the theater, they are right there on stage in front of the audience. A stage play relies heavily on dialogue and dialogue is what reveals character. The dialogue doesn't need to be written in the outline but thorough descriptions will help you develop each character's voice while you write the script.
Write the plot of the play. This is a short description of the scenes in each act, listed in the order they will appear. If the play is only one act this will just be a listing of the scenes. The scene descriptions should be listed as they appear in each act. Write the acts break into the outline so that you can see the entire play laid out, intermission and all.