Facial tissues such as Kleenex allways come in handy for things like this. Just don't use the kind that have the lotion in them. Besides being gross and smelling funny. they will add greasy splotches on the paper.
depending on the grain of the paper. hot or cold press you may have to make several passes. Build up the value slowly and don't scrub. Scrubbing will burnish the paper and make it shiney and harder to erase. You can smear around the pencil that is already there or make some shavings and dust the areas you want to shade before you rub it down.
Btw a soft cloth works just as well, but you don't have to wash a kleenex.
I hope that helps.
***have a nice day***