To add a controller extension and extend SAP Fiori, follow these steps:

1. Create a new controller extension file.

2. Implement the desired functionality in the extension, such as additional methods or event handlers.

3. Add the extension to the existing SAP Fiori app by referencing it in the app's configuration file.

4. Deploy the modified app to the SAP Fiori environment to make the extension effective.

5. 
   

Anubhav Online Training is a great interactive online resource for learning SAP UI5.