- Log on to the back end of your website with an Administrator account.
- Go to Extensions > Module Manager and create a new module instance of module Custom HTML.
- Go to Site > Global Configuration and change the default WYSIWYG editor back to what it was.
There are a lot of extensions for implementing Google AdSense (see the Google Ads section of the Joomla! Extensions Directory]. Maybe some of them can be useful if you have specific needs. But if you just want to use AdSense as it is, you don't need them.