Apply online with legal heir certificate or will of deceased along with ids's of deceased and legal heir. if income tax department is satisfied it will allow legal heir to file IT return. Legal heir. certificate is issued by district magistrate or appropriate authorities which differ in every state. If notarised will is present then it will also work.
Procure a application form from the revenue office or RI of your locality you can get online form @ cost of RS 60 from your state common service center or your state e governance site
then fill the form and makes sure you attached all the documents to it like self undertaking affidavit with stamp as well as notarized proof of identity death certificate proof of heir residence proof of death person resident date of birth certificate and all all documents required for it after this summit the application to your revenue officer or administrative officer of your area they they will approve your application by duly signed and their authority seal after this your application will goes to revenue authority he will enquiry or if needed do a local inspection if he is satisfied by his enquiry he forwarded your application to tahsildar he will issue a token for it and as per the specific date the application is scrutinize by him for final approval once the application approved you will get the certificate by sixteen day
Only notary is not taken in income tax department IT department took the certificate issued by court of law or say your civil court or local revenue authority that too fully singed and seal