1. Head over to the Firebase Console.
2. Create a new project or pick one you already have.
3. Add your Android or iOS app to the project and grab the config file (google-services.json for Android and GoogleService-Info.plist for iOS).
4. Drop that config file into your app.

For Android:

• In your build.gradle (Project level), add:

  ```gradle
  classpath 'com.google.gms:google-services:4.3.3'
  ```

• In your build.gradle (App level), add:

  ```gradle
  implementation 'com.google.firebase:firebase-messaging:21.0.1'
  apply plugin: 'com.google.gms.google-services'
  ```

For iOS:

• In your Podfile, add:

  ```ruby
  pod 'Firebase/Messaging'
  ```

• Run pod install to get those Firebase dependencies.

For Android:

• In your MainActivity's onCreate method, add:

  ```java
  FirebaseApp.initializeApp(this);
  ```

For iOS:

• Import Firebase in your AppDelegate:

  ```swift
  import Firebase
  ```

• Initialize Firebase in application:didFinishLaunchingWithOptions:

  ```swift
  FirebaseApp.configure()
  ```

1. Register for FCM in your app and get the token:

   For Android:

   • In your FirebaseMessagingService, add:

     ```java
     public class MyFirebaseMessagingService extends FirebaseMessagingService {

       @Override
       public void onNewToken(String token) {
           super.onNewToken(token);
           // Send token to your server or save it locally
       }
     
       @Override
       public void onMessageReceived(RemoteMessage remoteMessage) {
           super.onMessageReceived(remoteMessage);
           // Handle the received message
       }
     

     }
     ```

   For iOS:

   • Register for remote notifications in AppDelegate:

     ```swift
     UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .badge, .sound]) { granted, _ in
     guard granted else { return }
     DispatchQueue.main.async {
     UIApplication.shared.registerForRemoteNotifications()
     }
     }
     ```

1. Install Firebase CLI:

   ```bash
   npm install -g firebase-tools
   ```

2. Initialize Firebase Functions in your project:

   ```bash
   firebase init functions
   ```

3. Choose JavaScript or TypeScript as your language.

1. Open the index.js (or index.ts for TypeScript) file inside the functions folder.

2. Require the necessary modules and write a function that uses FCM to send a notification. Here's a sample:

   ```javascript
   const functions = require('firebase-functions');
   const admin = require('firebase-admin');
   admin.initializeApp();

   exports.sendPushNotification = functions.https.onRequest((req, res) => {
   const message = {
   notification: {
   title: req.body.title,
   body: req.body.body,
   },
   token: req.body.token,
   };
   
   admin.messaging().send(message)
   .then((response) => {
   res.status(200).send('Notification sent successfully: ' + response);
   })
   .catch((error) => {
   res.status(500).send('Error sending notification: ' + error);
   });
   });
   ```

1. Deploy the function using Firebase CLI:

   ```bash
   firebase deploy --only functions
   ```

2. Once deployed, the function can be triggered via an HTTPS request.

1. Send an HTTPS POST request from your app to trigger the Firebase function.

   Example with fetch in JavaScript:

   ```javascript
   const response = await fetch('YOUR_CLOUD_FUNCTION_URL', {
   method: 'POST',
   headers: {
   'Content-Type': 'application/json',
   },
   body: JSON.stringify({
   title: 'Hello!',
   body: 'This is a test notification.',
   token: 'TARGET_DEVICE_TOKEN',
   }),
   });

   const responseBody = await response.json();
   console.log(responseBody);
   ```

Note: Replace YOUR_CLOUD_FUNCTION_URL with the URL of your deployed function and TARGET_DEVICE_TOKEN with the token retrieved in Step 4.