Schedule a time to get your cast together. Have them wear costumes which are close to the type they will wear in the actual play. Take some shots of them in rehearsal and posed in character so you have several pictures to choose from.
Upload the pictures to a computer. Go through the pictures and select the one or two which best give the feeling of the play and will help sell it to an audience.
Open the picture you chose in a word processing or graphics program. Use the controls of your program to set the size of the poster.
Move the graphics to their locations and orientations in the poster margins you have set up.
Create a text box in your program at the top of the poster and type the name of the play in large noticeable letters. Type the name of the playwright directly underneath the title.
Create additional text boxes in your poster. Type the name of the director and actors, the dates of the show, the location of the theater, and any information regarding where the patrons can buy tickets.