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 somewhere in the viewport to place new track (no need to change any settings in the dialog).
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).
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. To 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 locomotive to add to the existing train.
Repeat previous step but click the last rolling stock to add at the end of the train.
If you want to change the direction of the rolling stock you place, press tab.