1. Go to Extensions > Templates > Templates
    Sub-theming Joomla template
  2. Select default or parent template, then click on Copy Template

    Sub-theme or clone a Joomla template version 3

  3. Copy the default en-GB.tpl.ini file, normally en-GB.tpl_templatename.ini
  4. Paste inside your new template directory templates/newtemplate/
  5. Open templateDetails.xml file (of new template) then update: Name and Description as needed
  6. Optional stuff:
    • Update a template thumbnail accordingly /templates/newtemplate/template_thumbnail.png
    • Create a custom CSS to contain overrides and custom styles; this is normally read by your template
      Customize and override Joomla duplicate template
      Custom and overriding Joomla template