Create an index.html file in your project directory.

Add Firebase SDKs to your index.html:
```html

```

Initialize Firebase in your JavaScript file (e.g., app.js):
```javascript
// Firebase configuration
const firebaseConfig = {
apiKey: "",
authDomain: ".firebaseapp.com",
projectId: "",
storageBucket: ".appspot.com",
messagingSenderId: "<YOUR_MESSAGING_SENDER_ID>",
appId: ""
};

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

Add a Google Sign-In button to your index.html:
```html
Sign in with Google
```

Implement the sign-in functionality in your app.js:
```javascript
// Get the sign-in button element
const signInBtn = document.getElementById('googleSignInBtn');

signInBtn.addEventListener('click', () => {
var provider = new firebase.auth.GoogleAuthProvider();

firebase.auth()
.signInWithPopup(provider)
.then((result) => {
var user = result.user;
console.log('User Info:', user);
})
.catch((error) => {
console.error('Error during sign-in:', error);
});
});
```