You can find Mosspaper's Widget here: https://app.mosspaper.com/d/#/widget. There are 2 sizes available for you to embed into your web site.
1) Log Into Joomla.
2) Select the page you would like to add the widget to.
3) Click on the "source code editor".
If you don't have a source code editor as an option, it's best to download a JCE- Joomla's Content Editor. Please go here for more details: https://www.joomlacontenteditor.net/. To download the editor, go here: https://www.joomlacontenteditor.net/downloads/editor.
4) Copy and Paste in the iframe code into the field.
5) Click Save. View the updated web site.