From the "Notifications" tab, you can easily schedule or send immediately notifications to users.
Click on "Add a notification", then fill in all the required fields.
When a notification is created, you can edit or delete it.
You can also send a test message to yourself by clicking on "Send me a test". However, you need to be connected to the same account of your mobile app and accepted to receive push notifications from the app.
Some notifications are activated by default, see: Which notifications are activated by default?