1. Switch mode
"Move" mode allow you to move the map around before you start. Press "Move" button to switch to "Measure" mode to lock the map and begin drawing.
Draw path on the map to start measuring distance.
In "Setting" menu (Wrench button on the bottom right), you could disable Auto-Reset to make it possible to keep switching between "Move" and "Measure" mode without losing progress.
Press "Share" button to share your result.
EX1. Change Unit
Press "Unit" button to switch between desired unit
Press "Search" button (Magnifier button) to move the map to desired location
You can revert progress by pressing either of these buttons.