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 scrathces

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 Ehiter 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?