Alright, let's dive into setting up Firebase Realtime Database. First things first, you need to create a Firebase project, grab your project credentials, and plug them into your app. If you're going for custom authentication, you'll need a secure server to handle token minting.

Head over to the Firebase console and sign in with your Google account. Click 'Add Project', give it a name, accept the terms, and hit 'Create project'. Easy peasy.

In the Firebase console, find 'Authentication' under the 'Develop' menu on the left. Click on the 'Sign-In Method' tab. You'll see a list of sign-in providers. Make sure to enable 'Anonymous' as one of them.

Now, get your Firebase project credentials and add them to your web app. Click the 'Web Setup' button on the Firebase console and copy the config object. Keep these credentials safe; they're unique to your project.

Firebase lets you create custom tokens on your secure server. These tokens are then used to authenticate the client device. In Node.js, you can use the createCustomToken(uid) method to create a custom token. The uid is the unique ID of the user. Make sure the Firebase Admin SDK is initialized with a service account to use this method.

On the client side, use the signInWithCustomToken(token).catch(function (error) method to authenticate with the custom token. Once authenticated, the Firebase Realtime Database is accessible.

After authentication, any database actions are tied to the user’s ID used when minting the token.

Database Security rules control how data is read and written. These rules live on Firebase.

To enable authenticated access, a sample rule-set might look like this:

Flexible: { "rules": { ".read": "auth != null", ".write": "auth != null" } }

This code means that users must be authenticated to read and write data. Using Firebase Database security rules properly helps keep your data secure and access controlled.

And there you have it! These steps show you how to secure Firebase Realtime Database with custom authentication and set up a user authentication system integrated with Firebase Realtime Database.