Open your Internet browser and go to SketchUp's Extension Warehouse.If you cannot install the plugin inside of SketchUp: Usually you will be able to use it without restarting SketchUp.Once you click 'Download' and follow the prompts, it will install itself.You will need to have a Gmail account and be signed in to download this plugin.Type in STL in the search box and then click on 'SketchUp STL by SketchUp Team.'.Open SketchUp and go to Window > Extension Warehouse.
