Adding Representative Assessee

https://www.incometax.gov.in/iec/foportal/help/authorize-signatory-faq

https://www.cadeveshthakur.com/income-tax/how-to-file-itr-in-case-of-deceased-assessee-register-yourself-as-representative/

https://www.incometaxindia.gov.in/Pages/tax-services/register-legal-heir.aspx

In order to register as  Representative Assessee on behalf of deceased assessse , the user has to first login using deceased assessse ID and password in IT e filing portal or his / her own credentials  ? Whose account to be used for logging in - deceased  person ID or representative asssesse ID ? 

https://www.incometax.gov.in/iec/foportal/help/authorize-signatory-faq

Please clarify . 

His own E-filing profile.