Navigate to the page you wish to modify

Click on Edit.

Click on the  sign below the title box.  This prompt will also appear between any existing web parts. 

Type Yammer in the Search box.

From the search results select the Conversations web part.

A Conversations pane will appear on the right of the window.

In Select conversation source, select one of the following:
Group: To show the most recent conversations posted in the selected group;
User:    To present the most recent conversations that a use have participated in;
Topic:  To show the most recent conversations tagged with a topic.
Feed:   To present the most recent conversations that show up for a user in Yammer when they click the Home button.
 

In Search for a source: enter the name of the group, user, or topic.

In the Number of conversations to show: option, move the slider to select between 1 and 12, based on how much space you want the web part to occupy on your page.

When you have completed adding the app click Save and Close.

To make your new web part visible to colleagues you will need to publish your changes.