It may just be me but I tend to start by envisioning a single scene and then building a story around it. For instance I got in my head the scene of a woman on a rugged cliff overlooking the sea giving birth in the middle of a storm.
That sparks some questions like -who is the women, where did she come from, why is she out here on this cliff, where is her family, what happens to the baby? Its the answer to those questions that become the setting - a harsh windswept peninsula jutting into a stormy sea (reminds me of Cornwall), the woman Wenna Kernowek, is from an isolated fishing village on lands owned by the Squire of Penruthen (Manor House on the Moors). The villagers are Wreckers who lure in passing ships (Lighthouse). Wennas baby is taken by the Storm Witch and will become a new sea hag. The Storm Witch is Patron of the local warlock/druid cult - Granite Moors, Rocky coast, isolated fishing village, offshore islands, dolmens, a lighthouse, a Manor house, a old shrine, a new chapel, the port at the base of the peninsula, surly locals, wreckers, customs agents, ghosts, bog hounds, selkies, hags, warlocks, druids, mermaids. NPCs: Storm Witch, Sea Hag (Morvah Kernowek), The Squire of Penruthan, Surly Locals
Then its deciding how the PCs will get involved (they’re on a ship lured in by wreckers? They’re Customs Agents from the local Magistrate? Something else?)
Once I’ve got those details I’ll get the Players to determine how they fit in and ask them to come up with their own NPC connections and any other motivations they might bring. I also give flexibility in scenes - essentially scene aspects so even if its not mentioned previously, if its reasonable to assume something is present the PCs can use it)