To quickly get started with flow maps, you can choose one of the flow map brush variants. They are available under the Real watercolor brush category. These brush variants enable flow maps by default. You can experiment and choose a different flow map to produce the effect you like.
Brushstrokes using three Real watercolor brush variants with the Fine Dots flow map selected
Brushstrokes using three Real watercolor brush variants with the Gravel flow map selected
You can also choose a flow map in the Flow Maps panel (Window Media Panels Flow Maps).