As the NUS Extra site no longer exists you will not be able to access your account here, even if your old NUS Extra card is still valid.
Using your NUS Extra card will work in the exact same way as if you had a TOTUM card. Simply log into the TOTUM App or website using the same login credentials as previously used.
If this is your first time logging into the TOTUM site then you may need to reset your password to ensure that your NUS details have been fully migrated over. You can do this via the link here.