Once you've clicked though to start working on your new blank tome, you should see an instruction to right-click on the root of the tome in order to add your first heading.

Once you have added a heading, you will see this on the screen in the main area. You will notice that as soon as you click off the text area, the content saves. You don't need to manually save as you go, any changes to text or headings or other content is automatically saved when you click elsewhere.