Create detailed character sketches. Mystery plays are character orientated, and the key to a good mystery play is the characters and their backgrounds. Add dimension to these characters. You'll need the sleuth, the victim and a cast of characters that will make good suspects. Decide whether your sleuth is going to be an amateur or a professional. Create backgrounds that make some of the personalities of you characters ambiguous. You want to make them suspicious.
Outline your story. In a murder mystery play, you will need to plant red herrings, which are false clues designed to throw the audience off, as well as real clues that give your audience a fair shot at figuring out the mystery. Pacing is essential to a good story line, as well as knowing what information you want to deliver to the audience and how much you want to reveal. Be sure to give the audience access to the same clues as your sleuth. The goal is to allow your audience to remain a step ahead of the sleuth without giving away anything until the end of the play.
Write a two- or three-page treatment for your play. You can use a treatment as an outline when you begin putting your story into a script format. This exercise allows you to work out the details of the crime in advance. Use your treatment or outline to help you with continuity issues. Consider working with your character sketches in front of you to help you maintain the personalities of your characters throughout the play.