Alright, let's get started by setting up Firebase in your project. Here's how:

• First, sign in to the Firebase Console.
• Click on "Add project," give your project a name, and hit "Continue."
• Stick with the default settings and click "Create Project." Now, just hang tight for a bit until the project is created.

Once Firebase is all set up, it's time to add Firebase Cloud Messaging (FCM) to your project.

• Click on the Settings icon (right next to Project Overview), and select "Project Settings."
• Head over to the 'Cloud Messaging' tab.
• Here, you'll find your 'Server Key' and 'Sender ID'. Jot these down, you'll need them soon.

Next up, we need to integrate the Firebase SDK into your app's code.

• Go to the left menu and select 'Cloud Messaging.'
• Click on 'Android' to get the Firebase SDK.
• Add the Firebase SDK to your AppModule gradle file and click on ‘Sync Now’ when you see the yellow bar notification.
• Initialize the Firebase application in your code.

Now, let's write a function to handle sending notifications.

• Add the code snippet into your app to send a new Message object. This includes the "to," "notification," and "data" payload.
• The "to" field should have the recipient's FCM token to target a specific user.
• The "notification" object will have the title and body of the notification.
• The "data" object is optional and can include custom key-value pairs for extra data.

Your app needs to ask for permission to send notifications.

• Include the code snippet that requests notification permissions.
• Once the user grants permission, they'll start receiving notifications.

When your app is in the background, the system UI handles FCM messages. But if the app is in the foreground, you'll need to handle the notifications yourself.

• Add an event listener in your app to listen for incoming messages.
• Include the code to handle notifications when the app is in the foreground, including displaying the messages.

Finally, it's time to test your application.

• Your app should now be ready to handle user-to-user notifications.
• Test it out by sending notifications to see if everything's working as expected.