You can basically use all the tools to make a running paint effect. I haven’t had much experience with it in real life, so I based these on pictures from google.
The easiest is just to draw it in with either of the pens, the marker or pencil, depending on what kind of look you want. If you’re drawing big, you can just use the watercolor brush.
Experiment. Learn how the different tools behave and what you can do with them. If you have a specific effect in mind, just try until you find something that works. I had no idea how to do this until you asked.