As the title says I really need help: I’m planning a horror themed one shot for Halloween, I have the basic idea and the setting but even if I’m not a new game master, I always mastered premade stories and I don’t really know of to create something from scratch

There will be 4 players and the sistem will Be Shiver, the setting (choosed by the players)is an Asylum After WWI and I’m planning to master an escape from a place where something terrible happened. The player can be Either Staff of the Asylum or Inmates that are trying to survive a very bad night, and I’d like to play with reality and hallucinations.

Do you have advices or suggestions for me, something to read to geat more ispiration or maybe tools to understand how to actualy write something?