Treat the headstone the way you would like yours to be treated - gently and with respect. Clean off any marks that you accidently put on the headstone. Soft brush Paper - Rice paper or white paper.
Rice paper is recommended.
Make sure the headstone is stable - Imagine how you you would feel if the headstone fell and broke. Clean the headstone with a soft brush to remove dirt and debris from the surface. If something is really stuck hard on the headstone, do not try and force it off as you may damage the headstone. Leave it - it adds character to your rubbing. Cut the paper to a size that is larger than the headstone and tape it down tightly so that it doesn't move.