• Head over to the Firebase Console.
• Click "Add project" and just follow the steps to create a new project.
• Once you're done, click on your new project to open up the dashboard.

• In the Firebase Console, go to "Project Overview".
• Click on the platform icon (iOS, Android, or Web) to add your app.
• Follow the instructions to register your app. For instance, you'll need the package name for Android or the Bundle Identifier for iOS.

• Download the GoogleService-Info.plist file after registering your iOS app.

• Open your iOS project in Xcode and add the GoogleService-Info.plist file.

• Add the Firebase CocoaPods to your Podfile:
  ```ruby
  pod 'Firebase/Core'
  pod 'Firebase/Messaging'
  ```

• Run pod install and open the .xcworkspace file.

• In your AppDelegate, import Firebase and configure it in the application:didFinishLaunchingWithOptions: method:
  ```swift
  import Firebase
  ...

  FirebaseApp.configure()
  ```

• Download the google-services.json file after registering your Android app.
• Place the google-services.json file in the app directory of your Android project.
• Add these dependencies to your build.gradle files:
• In the project-level build.gradle:
  ```groovy
  classpath 'com.google.gms:google-services:4.3.4'
  ```
• In the app-level build.gradle:
  ```groovy
  apply plugin: 'com.google.gms.google-services'
  implementation 'com.google.firebase:firebase-messaging:21.0.1'
  ```
• Sync your project with Gradle files.

For iOS:

• Ask for user permission for notifications in your AppDelegate:
  ```swift
  import UserNotifications
  ...

  UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .sound, .badge]) { (granted, error) in
  // Handle the result
  }

  application.registerForRemoteNotifications()
  ```

For Android:

• Create a service that extends FirebaseMessagingService to handle incoming messages:
  ```java
  public class MyFirebaseMessagingService extends FirebaseMessagingService {
  @Override
  public void onMessageReceived(RemoteMessage remoteMessage) {
  // Handle the message
  }
  }
  ```

• Go to the Firebase Console.
• Navigate to the Cloud Messaging section.
• Click "Send your first message".
• Fill in the message details and target either one of your app instances or a topic.
• Click "Send" to dispatch the notification.

Make sure the devices have registered tokens and are set up to receive notifications. Testing in both development and production environments is crucial to validate the entire setup. For detailed customization and further debugging, refer to Firebase documentation.