This extension is supposed to be as user intuitive as possible.
Hover a tool in the toolbar for a short description
and keep an eye on the status bar (at the bottom of the screen) for instructions when using the tools.
Draw a simple track loop
Select 'Add Track' tool.
Click the 'Junction' tab (video is from an older version of the plugin where 'Junction' was called 'From Scratch').
Click somewhere in the viewport to place new track.
Move a copy of the track about 50 m sideways (m is shortcut to move tool, press ctrl to move a copy).
Select 'Add Track' tool again.
Click an existing track to connect a new track to it.
Select 'Track Position' tool and click the newly created track.
Click the track end (light blue) located away from the connected track.
Click on the free standing track to connect the end to it (should form an arc).
Press enter.
Select the curved track.
Select 'Add Track' tool.
The tool should now try to add a copy of the selected curved track. Place the track so it connects the 2 loose track ends.
Now the track loop is finished! (but can of course be added to :) )
If you want the curves to look smother, right click them, open 'Track Properties' and change the number of segments. Too many segments slows down Sketchup. I typically use 24 for a 180° turn.
Add a Train
Select 'Add Rolling Stock' tool.
Select a locomotive and click on the tracks to insert it.
Select a wagon and click the existing train to add to it.
Repeat previous step.
If you want to change the direction of the rolling stock you place, press tab.