1. Go to the Dialogflow console and select Fulfillment from the left navigation menu.
2. Enable Inline Editor, paste the code from functions/index.js, then click Deploy.
3. Select Intents from the left navigation menu. Select the Default Welcome Intent intent, scroll down to the end of the page and click Fulfillment, check Use webhook and then click Save. This will allow you to have the welcome intent be a basic webhook intent to test.
4. Build out your agent and business logic by adding function handlers for Dialogflow actions.

To develop fulfillment functions, use the Firebase CLI.

The CLI allows you to create new functions, test them locally and manage deployments.