To plot a route, simply right click, where you want the route to start and select 'Start Route'
This will place the first waypoint of the route onto the map.
As you now move the mouse, a line will be drawn from your initial waypoint to the mouse location. Position the mouse where you want the next waypoint, and right click to place it.
The process now starts again, and you can plot your route by moving the mouse and placing waypoints.
If you wish to delete a waypoint you have just placed, use the delete key on the keyboard.
When you have finished your route, right click and select 'End Route'