Here is a quick example of a Sankey diagram setup.
In this example, we are looking at where people who leave San Francisco move to:
To get a Sankey diagram working, you need at least two dimensions and only one measure. For the migration example above, while my explore has four dimensions selected, I am hiding two of them from the visualization:
You can add more dimensions, based on what makes sense about the relationships you are trying to define. In this case, I added migration data from New York County, NY to see the common destinations people leaving New York and San Francisco go to. In this case, I unhid the “origin state” dimension so that now there are three dimensions (origin county, origin state, and destination county)
One thing to be aware of is that the Sankey reflects the order of dimensions as arranged in your “Data” tab. So if you want to see the flow from origin county -> state to destination state-> county, you will need to arrange your fields accordingly: