MMDTs can vary greatly depending on the theme, number of players, location, and MD's discretion. Here are some questions to ask yourself when writing your MMDT:
Characters & Plot
How deceitful do I want the characters to be? Are they allowed to lie?
In general, prohibiting characters from lying is only necessary if you need the MMDT to go by very quickly, or your actors are very young.
What is the protocol for asking questions out of character?
Are the characters allowed to kill one another?
If so, what's the protocol to allow dead characters to keep playing?
Will they be resurrected somehow? Will they be reincarnated as new characters? Keep in mind that it will be very boring for dead characters if they're not allowed to participate once they've died.
Clues
What combination of Challenges, Evidence, and Puzzle Pieces will I use? How will I ensure that all characters have an equal chance of figuring out the whole story?
How much control do you want to give players over clues? Do some players have clues as props that they must conceal/show other characters? Are they allowed to hide/conceal clues? Destroy clues?
Set-Up
Do I want this to be an Investigation or Perpetration MMDT? Or blended?
Perpetration MMDTs are good when you don't want the pretext for the event to be a murder investigation. Mafia: Silent Peace is an example of a Perpetration MMDT.
Investigation MMDTs are good if you want the characters to be suspicious of one another from the start. Terra Ignota is an example of Investigation MMDT.
Blended MMDTs only work if you establish that the characters are somehow able to kill one another during a murder investigation. The Secret of the Lily is an example of a Blended MMDT.
How will the characters recognize one another? Am I going to write full impressions sheets for everyone, or will I do character speeches instead? Will I use place cards and/or assigned seating?
There's an intro speech outline on the Blank Template page.