• Add/Create app with server key.
• You can find server key in your Firebase project settings.

• You can put device token, array list of device tokens or topic name in To input field.
• Click action is to open activity from Notification and it depends on how you named in AndroidManifest.
• If you added all the required fields, just press the floating action button.

• You can put your data json object in the JSON Object input field.
• Example : {"key1" : "value1" , "key2" : "value2"}

You can duplicate your recent pushed data messages, customize the values and push data messages again. Click on menu button (three dots) of pushed message and then Menu bottom sheet will appear. You can find Duplicate there.

Just follow the instructions at https://firebase.google.com/docs/cloud-messaging.