To add a pagination, click on the Add Block button and select the Page Break block. Alternatively, you can start typing
/pagebreak in a new paragraph block, then press enter.
Every block has its own block-specific controls that allow you to manipulate the block directly in the editor. The page break block is pretty straightforward so it only has the more options control.
You can add as many page breaks as you want. Each page break block adds a new number to the multi-page link that appears at the bottom of the page and enables you to split the content into more pages.
Clicking on the cog icon next to the publish button will toggle the visibility of this panel. While most blocks have specific options in the editor sidebar in addition to the options found in the block toolbar, the page break block does not have extra options.