Leigh Bardugo built the Grishaverse across Shadow and Bone and Six of Crows, and she's talked in detail about how she does it. She starts with people and plot, then works out how power moves through the world. The maps come with rules she can't undo later.

The Direct Answer
She builds from character and story first, then defines how power flows through the world before filling in the details.
She calls this the sense of power, the element she says falls into place first, per her Writer's Digest interview.

Power Comes Before the Map

Bardugo doesn't begin with a pile of invented history or a glossary of made-up words. She starts with her characters and the plot, and the first structural thing she settles is how power works. She's described this as the sense of power, the question of who holds it and how it moves through a society. That could be governmental power, personal power, or magical power. Once she knows that, she knows how her characters can behave and what they're allowed to do. In the Grishaverse, magic gets treated as a commodity, another form of power that people trade, protect, and go to war over.

The world-building that really falls into place first is what I always describe as the sense of power.

Leigh Bardugo, Writer's Digest (2018)

Maps Are a Promise You Can't Break

She's honest about the trade-off of drawing a map. The moment a map is printed, the geography is locked, and she can't quietly move a mountain range two books later without breaking faith with readers. That's part of why she commits so carefully to a world's shape early on. A map gives a series a spine, and it also takes options off the table.

Real places do a lot of the heavy lifting. Ravka draws on Russian folklore and Tsarist-era history, and Ketterdam, the canal-laced port city of Six of Crows, echoes the merchant world of Amsterdam. Bardugo has talked about keeping years of visual reference, including Pinterest boards, to hold the mood of a setting in her head while she writes.

How Six of Crows Made the World Bigger

Shadow and Bone is a chosen-one story about royalty and rare magic. Six of Crows opened a very different door into the same world, following expendable kids from the streets running a heist to survive. Bardugo has said that inventing that crew changed how she saw the whole universe and made it feel bigger and more believable, because readers finally got to see it from the gutter as well as the throne room.

For the daily mechanics behind all this, see the full Leigh Bardugo writing routine.

What You Can Steal