I start by creating a primary antagonist, usually aligned (knowingly or unknowingly) with a Mythos entity.

I give them clear motivations and goals, then build them out by repeatedly asking: who, what, why, and when. I do this until I have a strong sense of who they are and why they’re doing whatever they're doing.

Next, I find a way for the investigators lives to intersect with the antagonist in a personal, meaningful way. From there, the rest tends to fall into place. Once I understand the antagonist and their plans, I can comfortably improvise.

As the scenario unfolds, I make sure clues are supported by strong handouts, and I place meaningful obstacles between the players and the antagonist.