Namaste!If one has a desire to worship the Devi, it often shows even in ones dreams as visits to temples etc. Do not take up mantras based on someone telling you to do this over this or any other internet forum. Mantras are given in a particular order(krama) based on ones nature by a Guru. Keep doing what you can. A simple pancha upachar puja is good to start with. Gandha, Pushpa, Dhoopa, Deepa & naivedya is the right order. Be patient. At the right time, the Devi herself will appear as a Guru(in human form).
Your grandfather may have been given that mantra- you should wait until one is given to you.
If you can send me your birth details, I can try and tell you(through PM) when that blessed event will happen.