1. Go to the Firebase Console: Head over to Firebase Console and log in with your Google account.
2. Create a New Project: Click "Add project" and follow the steps to set up a new project.
3. Enable Authentication: In the Firebase Console, pick your project, then go to Authentication and select Sign-in method. Turn on the authentication providers you want (like Google, Email/Password, Facebook).

1. Add Firebase to Your Web App:

   • Go to project settings by clicking the gear icon near "Project Overview".
   • In the "Your apps" section, pick the icon for your platform (Web, Android, iOS).
   • Follow the setup instructions and jot down the Firebase config object, which includes the apiKey, authDomain, projectId, etc.

2. Install Firebase SDK:

   • For web apps: Install Firebase using CDN or npm.
     ```bash
     npm install firebase
     ```
   • For iOS/Android apps: Follow the specific platform setup instructions in the Firebase Console.

3. Initialize Firebase:

\`\`\`javascript
import firebase from 'firebase/app';
import 'firebase/auth';

const firebaseConfig = {
  apiKey: "YOUR_API_KEY",
  authDomain: "YOUR_AUTH_DOMAIN",
  projectId: "YOUR_PROJECT_ID",
  storageBucket: "YOUR_STORAGE_BUCKET",
  messagingSenderId: "YOUR_MESSAGING_SENDER\_ID",
  appId: "YOUR_APP_ID",
  measurementId: "YOUR_MEASUREMENT_ID"
};

// Initialize Firebase
firebase.initializeApp(firebaseConfig);
\`\`\`

1. Sign Up/Sign In User using Firebase Auth:

   • Example using Email/Password method:
     ```javascript
     const email = "user@example.com";
     const password = "password123";

   firebase.auth().createUserWithEmailAndPassword(email, password)
   .then((userCredential) => {
   // Successfully created user
   var user = userCredential.user;
   })
   .catch((error) => {
   var errorCode = error.code;
   var errorMessage = error.message;
   });

   firebase.auth().signInWithEmailAndPassword(email, password)
   .then((userCredential) => {
   // Successfully signed in
   var user = userCredential.user;
   })
   .catch((error) => {
   var errorCode = error.code;
   var errorMessage = error.message;
   });
   ```

2. Retrieve User Token:
   ```javascript
   firebase.auth().currentUser.getIdToken(/_ forceRefresh _/ true).then(function(idToken) {
   // Send token to your backend via HTTPS
   }).catch(function(error) {
   // Handle error
   });
   ```

1. Create Backend API: Set up an HTTP server using a backend framework you like (Node.js, Express, Django, etc.).

2. Install Firebase Admin SDK: You'll need this to verify Firebase tokens in your backend.

   • For Node.js:
     ```bash
     npm install firebase-admin
     ```

3. Initialize Firebase Admin SDK: Initialize the Firebase Admin SDK in your backend.
   ```javascript
   const admin = require('firebase-admin');
   const serviceAccount = require('path/to/serviceAccountKey.json');

   admin.initializeApp({
   credential: admin.credential.cert(serviceAccount)
   });
   ```

1. Create Middleware to Authenticate Requests:
   ```javascript
   function authenticateToken(req, res, next) {
   const idToken = req.header('Authorization').split('Bearer ')[1];

   admin.auth().verifyIdToken(idToken)
   .then((decodedToken) => {
   req.user = decodedToken;
   next();
   }).catch((error) => {
   res.status(401).send('Unauthorized');
   });
   }
   ```

2. Protect Routes with Middleware:
   ```javascript
   const express = require('express');
   const app = express();

   app.use(express.json());

   app.post('/someProtectedRoute', authenticateToken, (req, res) => {
   res.send('This is a protected route');
   });

   app.listen(3000, () => {
   console.log('Server started on port 3000');
   });
   ```

1. Test Authentication in Your App:

   • Make sure you can sign up and log in users through your frontend.
   • Check that the ID token is being sent to your backend correctly.

2. Verify Backend Security:

   • Ensure that only authenticated requests can access protected routes.
   • Verify that the backend processes ID tokens correctly and grants access based on the token information.

This approach ensures that your application maintains a secure authentication flow, leveraging Firebase Authentication for user management while utilizing a custom backend to handle sensitive operations and business logic.