To create your KML, we suggest downloading Google Earth Pro from here.
Search for the site you would like to create the KML for using the search bar in the top left.
In the toolbar at the top of the screen look for the add polygon button and press it which opens another window for the application. In this window, give your polygon a name. Then go to the Style/Colour option and give your polygon outline a colour and make the shape transparent to make it easier to create an accurate polygon.
Next, draw the boundaries of the site by clicking on the corner points of the site until the polygon accurately depicts the area of interest of the site. You can adjust the position of the corner points by clicking on them and dragging them to the new location on the map. Alternatively, you can delete them by clicking on the point you wish to delete and pressing the delete button/backspace on your keyboard.
Finalise the site boundaries by pressing OK, then find the name of your site under the 'Places' window on the left. Right-click it and select 'Save Place As' choose the destination you would like to save it to and make sure you save it as a KML. Do not save it as a KMZ.