by matthias.s on WordPress.org
TinyMCE Page Break Button adds a Page Break Button to the TinyMCE Menu for using the Nextpage-Tag in posts.

The standard TinyMCE before you have installed and activated the TinyMCE Page Break Button
This zero configuration Plugin adds a Page Break Button to TinyMCE Menu for using the Nextpage-Tag in posts.
Only install and activate. That’s all.
Your Theme must support the Nextpage-Tag.