In iPad Settings, go to General, then Transfer or Reset, then "Erase All Contents and Settings". The next thing you see ( after confirmation) is the welcome screen as if just out of the box.
Also: If shipping it you might want to power off by holding top and bottom buttons until you see the "slide to power off" slider. I recently sold my iPad 2, cleared it to factory settings as described and powered off. Happy new owner received it with charge, ready to configure.
Late note:
If FindMy is enabled, you must disable this first [meaning you must have access to its iCloud account] or the new owner will not be able to use it, it will still be locked to you.