Instructions
Create a cast of characters equal to the number of guests you intend to have play. Your cast will be one larger, to include the character of the murdered victim.
Outline what happened as if you were going to write this as a story instead. In the outline, know what all the other characters were doing before, during and after the murder. Plot out the murder carefully. Remember to make it a little complex so that there will be plenty of red herrings.
Go through your outline and construct a list of clues for each character. Each character should have things they have seen or heard, rumors they will spread and an alibi. In the case of the murderer, he will spread false clues and be the only character allowed to lie.
Portion out the clues to be delivered in specific acts. Many murder mystery games are played in three acts. Decide which clues should come out in each act, giving some clues to each of the characters. Clues should be small at first and should build up in importance by the last act.
Write all this information down on individual character sheets. On each sheet, give the player a brief bio about her character so that she can portray the character in the game. Be sure to tell the murderer up front that she is the murderer. After the bio, give the clues to be disclosed for each act.
Write up the scenario for everyone to read or hear at the time of play. In this, you will introduce the basic setting, including what was going on at the time of the murder. Describe the scene of the crime. With this, you have the basic setup. Once you have all these parts written, you'll have your own murder mystery game which you can play with friends.