Below is what you will have in your html :
Object gx holds this KML file . You just have to add it as an overlay. Remember, your kml files have to on some public server and not localhost.
The content of KML is as follows :
In The KML file, the first thing that should be taken care of is namespace or xmlns="http://www.opengis.net/kml/2.2" which, if is wrong, Google won't load that kml. Each object can be displayed using Placemark which has a name and the object definition (Polygon in my case). Additionally you can add markers, polyline etc.