Begin with a one-sentence decision statement and a definition of done. Then list the data sources, known constraints, people who will use the result, and the date it needs to be useful.
Break the work into checkpoints that produce something reviewable. An early sketch of the output often reveals unclear requirements sooner than a polished final dashboard.
Leave room for discovery. Data projects rarely follow the first plan exactly, and a visible decision log helps everyone understand why the scope changed.