First you will have to download the add-on by navigating to the Template Gallery Download Page. Select Free where you will be asked to continue and allow permissions.
Then you will be able to run the add-on by navigating to a Google Docs document and selecting the Add-ons tab at the top, and select Template Gallery and within that menu select Browse Templates.
There a screen will appear with a categorized listing of the various kinds of templates that can be used.
Once you find and select a template that you like, select the button labeled Copy to Google Drive. The File Type information below this button will let you know if the template you selected is a template for Google Docs or Sheets.
After copying to Google Drive, the same menu will appear, this time with the blue button saying Open File. Selecting that will open that template in a new tab. You can now begin editing the template!