How to send custom notifications using process builder

In Summer 19 Salesforce provides the option to send custom notification from process builder.

• Custom notifications allow you to send important information to your users via push to mobile or desktop alerts. 
• you can fully customize the notifications to inform users of record information, changes, and updates. 
• Custom notifications will bring relevant information to your users at the right time.

Note:
Before Customizing the Custom Notification you need to define custom notification type in notification types.
Path:
From Setup, enter Notification builder in the Quick Find box, and then Select Notification types

Then Create a new custom notification type and you can define where we need to show notification Desktop or Mobile

Once you created the Notification type, now ready to create process.

Example: 

In this example, I created the process builder and defined an object as Opportunity, when the record is created or edited it will fire.

I just checked the criteria as whenever the opportunity stage is 'Closed Won'

Select the action type is 'Send Custom Notification'
Select the notification type you created in notification types Select the Notification Recipient such as User, Owner, Group, and Queue(who wants receive the notifications)
Define the notification title and notification body.

Successfully completed the Process creation now it is ready for testing.
To test the process just close any opportunity with stage name is 'Closed Won'.
Result:

Limitations

• You can create up to 500 custom notification types.
• Each notification can have up to 10,000 users as recipients. However, you can add an action to the same process within Process Builder or to the same flow in Flow Builder to have more recipients.
• Your org saves your most recent 1 million custom notifications for view in notification trays. Your org can save up to 1.2 million custom notifications, but it trims the amount to the most recent 1 million notifications when you reach the 1.2 million limit.
• An org can execute up to 10,000 notification actions per hour. When you exceed this limit, no more notifications are sent in that hour, and all unsent notifications are lost. Notification actions resume in the next hour
• For example, your notification action processes are triggered 10,250 times between 4:00 and 5:00. Salesforce executes the first 10,000 of those actions. The remaining 250 notifications aren’t sent and are lost. Salesforce begins executing notification actions again at 5:01.
• Custom notification title and body fields support plain text only.

Resources
https://releasenotes.docs.salesforce.com/en-us/summer19/release-notes/rn_forcecom_flow_notificationbuilder.htm