First of all you have to make sure that your Sony Smart TV is compatible with Rakuten TV.
Follow this link to check if your device is included on the list of compatible devices:
Make sure that you Sony Smart TV is connected to the Internet.
Once you have confirmed that your device is compatible with Rakuten TV, verify if it's connected to the Internet. To do this you should connect your Sony Smart TV to a router or modem using a connection through an Internet provider.
Turn on your Sony Smart TV.
Using your remote control look for the button "Home" and press it.
Straightaway select "Internet Content".
Look for the Rakuten TV icon and press it.
If the Internet connection is correct, the Rakuten TV navigation menu will show up.
If the Rakuten TV icon doesn't show up, go to the "Settings" in the main menu. There go to "Network" and select the option "Update Internet Content". Once updated, the Rakuten TV icon will appear in the "Internet Content" menu.
Congratulations! Now you can enjoy Rakuten TV in your Sony Smart TV.