Select from the menu and click on the page where you want to create a sub-page.
This page will open on your screen.
Then click Add a New Page. Enter a name for the sub-page in the relevant space (Page name).
Click Add Page.
If you click, now, on the page that you had initially selected in the menu, it will also appear its sub-page. In this way you can create as many sub-pages you wish on a page.
Menu pages without sub-pages bear a red dot in front, while pages with sub-pages have a red arrow in front.