1. Head over to the Firebase Console.
2. Click on "Add Project."
3. Type in your project name and follow the steps to set up your Firebase project.

For Android:

1. In the Firebase Console, pick your project.
2. Click on the Android icon to add an app.
3. Register your app with your package name.
4. Download the google-services.json file.
5. Move the google-services.json file into your app/ directory.
6. Add Firebase SDK dependencies to your build.gradle files.

For iOS:

1. In the Firebase Console, select your project.
2. Click on the iOS icon to add an app.
3. Register your app with your iOS bundle ID.
4. Download the GoogleService-Info.plist file.
5. Move the GoogleService-Info.plist file into your Xcode project navigator.
6. Add Firebase SDK dependencies to your Podfile.

For Android:

1. Open your MainActivity.java or MainActivity.kt.

2. Add the initialization code inside the onCreate method:
   ```java
   // Java
   import com.google.firebase.analytics.FirebaseAnalytics;
   private FirebaseAnalytics mFirebaseAnalytics;

   @Override
   protected void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   setContentView(R.layout.activity_main);
   
   // Obtain the FirebaseAnalytics instance.
   mFirebaseAnalytics = FirebaseAnalytics.getInstance(this);
   }
   ```

For iOS:

1. Open your AppDelegate.swift or AppDelegate.m file.

2. Add the following code to initialize Firebase:
   ```swift
   // Swift
   import Firebase

   @UIApplicationMain
   class AppDelegate: UIResponder, UIApplicationDelegate {
   var window: UIWindow?
   
   func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
   FirebaseApp.configure()
   return true
   }
   }
   ```

For Android:

1. To log custom events, use the logEvent method:
   ```java
   Bundle params = new Bundle();
   params.putString("item_name", "example_item");
   params.putString("user_id", "123456");
   mFirebaseAnalytics.logEvent("event_name", params);
   ```

For iOS:

1. To log custom events, use the logEvent method:
   ```swift
   Analytics.logEvent("event_name", parameters: [
   "item_name": "example_item" as NSObject,
   "user_id": "123456" as NSObject
   ])
   ```

For Android:

1. To set user properties, use the setUserProperty method:
   ```java
   mFirebaseAnalytics.setUserProperty("favorite_food", "pizza");
   ```

For iOS:

1. To set user properties, use the setUserProperty method:
   ```swift
   Analytics.setUserProperty("favorite_food", forName: "pizza")
   ```

1. Go to the Firebase Console.
2. Select your project.
3. Head to the "Analytics" section.
4. Check out your user activity data under “Events,” “Overview,” and other available tabs.