What is the ruling of the Sharia on writing a supplication, said to be for liberation from Hellfire, with saffron, musk, and rosewater, and sprinkling it on the shroud of the deceased?

Writing supplications with saffron or other substances and sprinkling them on the deceased's shroud has no legal basis (in Islamic law) and is considered an innovation (bid'ah). One should limit themselves to religiously sanctioned acts, such as supplicating for the deceased and giving charity on their behalf.