• Head over to the Firebase Console. Create a new project or just open one you already have.
• Add your app to the Firebase project. Pick your platform (iOS, Android, or Web) and follow the setup steps.
• Download the google-services.json file (for Android) or GoogleService-Info.plist file (for iOS). Put it in the right spot in your project.

Android

• Add the Firebase SDK to your Android project by updating the build.gradle files:
  ```groovy
  // Project-level build.gradle
  buildscript {
  dependencies {
  classpath 'com.google.gms:google-services:4.3.10'
  }
  }

  // App-level build.gradle
  dependencies {
  implementation 'com.google.firebase:firebase-config:21.0.1'
  }

  apply plugin: 'com.google.gms.google-services'
  ```

iOS

• Add the Firebase SDK to your iOS project using CocoaPods. Update your Podfile:
  ```ruby
  pod 'Firebase/Analytics'
  pod 'Firebase/RemoteConfig'
  ```
• Run pod install to integrate the Firebase SDK.

Web

• Add Firebase SDK to your web project by including the scripts in your index.html or module files.
  ```html ```

Android

• In your Application class or MainActivity:
  ```java
  FirebaseApp.initializeApp(this);
  ```

iOS

• In your AppDelegate file:
  ```swift
  FirebaseApp.configure()
  ```

Web

• Initialize Firebase in your JavaScript file:
  ```javascript
  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"
  };

  const app = firebase.initializeApp(firebaseConfig);
  const remoteConfig = firebase.remoteConfig();
  ```

• Go to the Firebase Console and select your project.
• Navigate to Remote Config from the left menu.
• Click on Create Config.
• Add parameters for your feature flags. For example, add feature_new_ui as a boolean type and set its value to true or false.

Android

• In your Activity or Fragment:
  ```java
  FirebaseRemoteConfig mFirebaseRemoteConfig = FirebaseRemoteConfig.getInstance();
  mFirebaseRemoteConfig.fetchAndActivate()
  .addOnCompleteListener(this, task -> {
  if (task.isSuccessful()) {
  boolean newUI = mFirebaseRemoteConfig.getBoolean("feature_new_ui");
  if (newUI) {
  // Enable new UI feature
  }
  }
  });
  ```

iOS

• In your ViewController:
  ```swift
  let remoteConfig = RemoteConfig.remoteConfig()
  remoteConfig.fetch { (status, error) in
  if status == .success {
  remoteConfig.activate { (changed, error) in
  let newUI = remoteConfig["feature_new_ui"].boolValue
  if (newUI) {
  // Enable new UI feature
  }
  }
  }
  }
  ```

Web

• In your JavaScript file:
  ```javascript
  remoteConfig.fetchAndActivate()
  .then(() => {
  const newUI = remoteConfig.getValue('feature_new_ui').asBoolean();
  if (newUI) {
  // Enable new UI feature
  }
  })
  .catch((err) => {
  console.error("Error fetching and activating Remote Config:", err);
  });
  ```

• Test the changes by toggling the flags in the Firebase Console.
• Validate the behavior of your app with the new feature enabled or disabled.
• Iterate and update the feature flags as needed in the production environment without requiring an app update.