...
Selected the relevant Polygon from the listed polygons.
In the toolbar located above the top viewport, select the cross-section button.
Click and hold the left mouse button inside the top viewport and move the cross-section arrow across the selected polygon.
The cross-section will then be displayed in the bottom viewport.
Use the Step Size box and blue up and down arrows to change the step size of the cross-section arrow and move it across the polygon.
...
Use the scroll wheel to scale the view in the Cross-Section panel, or hold it down to move the cross-section arrow.
Once the desired view has been established, select the black downwards pointing arrowhead located in the Cross-Section column.
...
Select the “Save” option.
...
Clearing the Cross-Section
...