If you don't know how, don't do it! You might cause more harm then good.
If you don't own the monument or are a member of the immediate family you shouldn't try to clean.
Some helpful links for proper cleaning.
http://www.gravestonepreservation.info/articles/cleaning-gravestones-monuments-stone-sculptures
http://www.ctgravestones.com/Conservation/cleaning.htm
Gravestone rubbing can also cause damage if not done properly.