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.