Ї пишџ карандашћм/рџчкой. I write with a pencil/pen.  
  Ї љм сџп лћжкой. I eat soup with a spoon.  
Note that no preposition is used in these phrases. If you do use the preposition с (a common mistake made by speakers of English), the meaning of the phrase will certainly be different from what you intend to say. Compare:
  Ї љм сџп со сметђной. I eat soup with sour cream.