well first you need a mystery. then when you show the clues make your veiwer not think any of it makes sense to the story so:
get all the clues!
now muddle them up so they seem un conectedd.
now put it into your script!!!
What Makes A Good Mystery? Edit
well here are some ideas for a mestery:
a good murder mystery is all ways a good classic if you can maby add a difference to the other murder mystery movies by maby adding:
A Nonhuman murderer Edit
Maybe a classic mystery story with a sifi twist!
Make It Funny! Edit
what if the victim was a snow man???
feel free to add your ideas here and in comments!