A Mystery Board Game is a game in which players compete to be the first to solve a who-done-it. Typically, these games will involve using cards and dice. They usually have the same kind of plot line as well-known mystery stories like those by Agatha Christie or about Sherlock Holmes. The players receive clues and try to figure out the identity of a murderer or other criminal. You can design your own mystery board game with a little imagination and some creative ideas for play.
Come up with some ideas for a detective story or mystery. You will need a lot of them--enough to keep your players occupied through multiple games. You can use the same general idea for each mystery, but you will want to vary the specific story lines. You will also need to make up some characters. These may be the same each time you play, or they can be different depending on the game. You will also need to make up a game board. This can simply be a piece of cardboard on which you map out locations and maybe paths between the different locations. Think of each location as a single setting where some of your action takes place. These can be rooms in a house, or even different buildings or outdoor locations.
Take some blank cards--index cards or pieces of cardboard will do fine. Write some instructions on these cards. These instructions should be pieces of the mystery and also things that the players must do as they move around the board. You can use dice to determine the players' moves. Maybe they have to choose a card when they land on a particular space on the board. Possibly they have to collect different cards to assemble the pieces of the puzzle. A mystery board game can use a few different decks of cards, each with its own purpose. You might even use the cards to describe characters involved in the story. You decorate them with pictures of your characters. Create some game pieces out of paper or pieces of metal or plastic. If you are artistic you can make them look like a detective in one of your stories.
Try to involve the players directly. Make them a part of the story. One of them could be the killer or thief. A roll of the dice, or a chance card could determine which of them is a criminal master mind. Alternatively, the cards could tell them what they have to do to accomplish their crime. A good idea would be for players to hold onto clues that are unknown to other players. Possibly players could make deals with each other to find out information or obtain these cards. To make the game more complex, require your players to use and purchase weapons and disguises. Think about whether they have to use money in your mystery board game--play money of course--or need to accumulate points to perform certain activities or uncover clues.
Stick to a general theme. If your setting is an old English manor house make your stories fit that setting. Keep to a single time period. Do not make your mysteries too confusing too follow. Cleverness is good but tales that are too convoluted will only turn people off. You want people to have fun, not push your game aside. A good mystery board game should be like a good mystery novel. You should be able to solve the mystery if only you use your mind and pay attention to the clues. The solution may not--should not--be obvious but it should follow naturally from the clues and moves in your game.