To post chatter, your organization needs to have chatter enabled in the environment. Once it is enabled this action can be used to post on the chatter, the feed will appear as soon as the user who started the process, by creating or editing the record. 

The feed can be posted on user’s feed, to a group in chatter or the record’s feed that started the process. 

To post to the user’s chatter feed we have to do the following. 

For this example we will use the process builder that we previously created : Opportunity won

1. Clone the existing Process and create a new version.
2. On the First Criteria node ‘Opportunity Won’, click on the ‘+ Add Action’ button. 
3. Select Post to chatter as the action type.
4. Define the Name of the Action :’Post to User’
5. In the picklist ‘Post To’ select: User

6. Then use the selection to define which user should the feed be posted to, in this case we chose the user Id of the last ,modified by user. 
7. Fill out the message body that you want to post to the feed, here you can also use the merge field to display value from the record that is getting created or edited.

8. Once the above changes are made, then save the action and activate the process builder. 
9. Below is the feed on the user’s page. 

To Post to a chatter group, you have to select the Chatter group in the Post TO selection picklist and do the same as above. 

To Post to a records feed you have to below. 

1. Clone the existing Process and create a new version.
2. On the First Criteria node ‘Opportunity Won’, click on the ‘+ Add Action’ button. 
3. Select Post to chatter as the action type.
4. Define the Name of the Action :’Post to User’
5. In the picklist ‘Post To’ select: This Record
6. Then next all you have to do is define the body of the feed. 
7. The overall action should look as below. 

8. Below is how the feed appears on the record’s page. 

For More Examples refer : Automate you Business Process