Make sure you have Node.js installed for server-side scripting.

Install the Firebase Admin SDK using npm:
```bash
npm install firebase-admin --save
```

Initialize Firebase Admin in your server code:
```javascript
const admin = require('firebase-admin');
const serviceAccount = require('path-to-serviceAccountKey.json');

admin.initializeApp({
credential: admin.credential.cert(serviceAccount)
});
```

Once the Firebase Admin SDK is up and running, generate Firebase custom tokens on your server:
```javascript
const uid = 'some-unique-user-id';

admin.auth().createCustomToken(uid)
.then((customToken) => {
// Send token back to client or use it as needed
})
.catch((error) => {
console.log("Error creating custom token:", error);
});
```

If you want, you can include custom claims:
```javascript
const additionalClaims = {
premiumAccount: true
};

admin.auth().createCustomToken(uid, additionalClaims)
.then((customToken) => {
// Send token back to client or use it as needed
})
.catch((error) => {
console.log("Error creating custom token:", error);
});
```

Once authenticated, Firebase gives you user information for subsequent operations.

Access the authenticated user details:
```javascript
const currentUser = firebase.auth().currentUser;

if (currentUser) {
console.log("User ID:", currentUser.uid);
} else {
console.log("No user is signed in.");
}
```

Use the authenticated user for application-specific functionalities like accessing the database, Storage, etc., with the security rules applied based on user authentication.