Transfer of property after death

The residential flat in question was originally purchased 20 years back. 25% share in my grandfather's name 25% share in my grandmother's name and 50% in my father's name. My grandmother is still alive.. My grandfather died 6 years back. He had made a will where he had entitled my father and my mother to take ownership of his 25% share in the said property after his death. My father has 2 sisters, 2 brothers out of which one brother is dead.. The brother who died has 2 sons and a wife left behind.. Now i want my grandfathers share be transferred to my father and my mothera name..My questions are 1. Who's all NOC do we require to transfer the share certificate and the property share in my mother's and father's name? 2. NOC shall be on stamp paper of what denomination? 3. Do I have to purchase seperate stamp papers for seperate people or I can take all of their sign on one stamp paper? 4. Does the NOC have to be notarized or registered? If yes where and by whom? 5. Any other relevant step which I need to take?