Once you have created a page using Add It, you should first add appropriate tags in the metadata form by following this article: Editing Atlas page metadata
With your page tagged you can start to add content. Atlas pages can contain text, images, embedded content such as video, and web parts which can be configured for many different uses. Atlas supports both out of the box SharePoint web parts and Atlas-specific web parts provided as part of the product.
To learn about adding and configuring web parts, see this article: Managing web parts on Atlas pages
Please note that Atlas web parts are only supported inside web part zones on SharePoint Classic pages. See more information here: Atlas Web Parts are not supported outside the defined Web Part Zones
In the main content area, SharePoint provides a simple editor featuring the Microsoft Office ribbon which should be familiar to users of Microsoft Word, Excel etc. This can be used to add the static content you want on the page. To add content to the main content area, simply click into it to start typing. This will also load the ribbon giving you access to Microsoft Word-like functionality to insert other content and formatting:
Your page can be as long as you like, but consider making pages as short as you can to keep people's attention. Also, if you add web parts at the bottom of the page, ensure your page is not so long that they are not used.