Once you enable plg_content_rssfactory from the Plugin Manager page of Joomla you will be able to use the Content Plugin similar to this:
{com_rssfactory relevantStories nrFeeds=[3]}
The format used for tokens is this: tokenName=[value1, value2, value3]
In the example shown above "relevantStories" and "nrFeeds" are token names while "3" is the value, in this case representing the number of feeds to be displayed.
Here is a list of the available tokens that can be used:
{com_rssfactory nrFeeds=[3] searchTerm=[sport,tennis,nadal]}
Displays stories from 3 feeds that contain one of the search terms: "sport", "tennis" or "nadal", you could use
{com_rssfactory nrStories=[1] relevantStories}
Displays stories relevant to the current article Joomla category, only one story for each feed.
{com_rssfactory categoryId=[4] displayMethod=slider}
Displays stories of feeds using a slider format from the category which has the ID "4".
(!) Documentation based on RSS Factory and RSS Factory PRO version 4.2.4