Instructions
Enter Twilight Town when it comes back on to the world map. Go to The World That Never Was and enter the skyscraper.
Complete the boss fight "Final Mix." Obtain a key blade that increases your chances of converting into Final Form.
Level up your Final Form by going to the Underdome. Enter the Cerberus Cup and select "Master Form." If you don't get Final Form, push the "Start" button, and then select "Retry." Keep doing this until you're in Final Form.
Keep your drive gauge full. You can do that in the Cerberus Cup, but you can also do it by going to any save point while you're in any drive form and exiting to the world map. Return to the same save point and your gauge should be full again.