Before we get started, you should remember the definition of a locked-room mystery: a murder in which the room where the victim is located is completely sealed off (usually from the inside). One of ACD's most famous locked-room mysteries is
The Adventure of the Speckled Band.
Enough of that, though- let's get on with the mystery!
1. Choose Your How
All good murder mysteries have an interesting story. How was the person murdered? Even if you don't have some grand scheme in mind, you should have a clear idea of how your victim was killed.
2. Keep it Creative
Every published locked-room mystery has its own twist- why should yours be any different? Whether it’s the murderer, victim, weapon, or way in which they were murdered, it’s always good to have something unique that the reader will remember.
3. Draw it Out
Make sure you know your murder forwards, backwards, and inside-out. If you need to, draw out a diagram of the room and write out every step every person takes. Mark out the steps with a friend (if they’re willing, of course). Do whatever it takes short of committing the crime itself to get it cemented in your head.
4. Know Your Detective
Behind every mystery is a great detective ready to take it on. What type of person is your detective? If your detective's not an interesting person, then readers are just reading to solve the crime, and won't care about anything not related to the crime. Your detective should be a 3D character with strengths and flaws (just like all your others).
Try out these tips with your own locked in murder story. Let us know your advice in the comments!