📝 Notepad

Create a free Firebase project
Go to console.firebase.google.com → Add project → give it any name → Continue (disable Analytics if you like) → Create project.

Add a Web app & copy the config
In your project, click the </> web icon → Register app → copy the firebaseConfig object.

Enable Firestore Database
In the left sidebar: Build → Firestore Database → Create database → Start in test mode → Next → Enable.

Enable Google Sign-in
Build → Authentication → Get started → Google → Enable → Save.

Set Firestore security rules (keeps your data private)
Firestore → Rules tab → replace the rules with:

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId}/{document=**} {
      allow read, write: if request.auth != null
        && request.auth.uid == userId;
    }
  }
}

Click Publish.