It’s helpful to sprinkle some arrows onto your model. Right-click a pipe, then select “Arrow on Stream”:
(Note the shortcut shown on the menu: you can also hit ^ on your keyboard).
Your mouse pointer will change to:
Now wherever you click on a pipe, an arrow will appear.
While you are still in “arrow” mode, if you click on the tip of the arrow, it will disappear.
Hit the Esc key to exit “arrow” mode.
After you are out of “arrow” mode, you can move an arrow by clicking near the tip and sliding it to a new position.
Here’s a 1.5 minute showing these actions: adding, deleting, and moving arrows.