To activate Disney+ on a Smart TV or Roku, start by downloading the Disney+ app from your device's app store. Once installed, open the app and log in to your account. A unique activation code will appear on the screen. Visit disneyplus.com/begin on a web browser and enter the code. After submitting the code, your Disney+ account will be synced with your Smart TV or Roku. You can now start streaming Disney+ content directly on your device. Ensure your device is connected to the internet for seamless activation.