Log into your Firebase account and select the correct project.
On the right hand toolbar you will see notifications:
(In here you will see past messages sent and their status)
Click 'New Message' and you will be taken to the below screen:
'Message Text' is the body of the messages, the information you would like your users to see.
'Message label' is the name you would give to the push, this isn't visible to the users on the push, it's to help identify between messages.
'Delivery date' as you can set to send now, or set to send some point in the future, this is how you can schedule your push notifications.
'Target' as you can target different groups, i.e. Android/IOS. If you select IOS, and click 'target another app' and select Android, the push notifications will go to both groups.
'Advanced options' - recommended:
'Title' can be left blank
'Custom data' can be left blank
'Priority' will decide how high in the users notifications this push will appear, usually always set to high
'Expires' means if a device is offline (or not connected to wifi/mobile data), Firebase will keep trying to send it to the device for up to X amount. It is recommended to set to 1/2 days - as if the push is about a new edition, the push will be out of date if the user receives the push in 4 weeks time.
When you are happy with the message and the options, select 'send message' and you will be taken back to the overview of all push notifications sent.