Old Ipad mini

If you are able to access iPad settings, a full erase and reset is just a few taps away…

Settings > General > Reset or Transfer iPad > Erase All Content & Settings

Otherwise, you can restore the iPad to factory settings - which will fully erase the iPad and install a fresh copy of the most recent version of iOS/iPadOS supported by your iPad:

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

To do so, you will require access to a either a Windows PC (with iTunes installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of MacOS). Ensure that your installed version of iTunes has been updated to the most recent version; the essential device drivers are included.

Download and use iTunes for Windows – Apple Support

Update to the latest version of iTunes – Apple Support

Follow the steps in the support document you posted from.

If you are having a specific issue, explain what that is.