If you are asking how to decorate a cake without buying anything, than for writing, put icing into a plastic bag, squeeze it into on corner and snip a small hole in it.
Or you could just make a simple border with a larger hole than for writing in a plastic baggie (like the first answerer told you) then put little candies in it, you could use jelly beans, red hots, sprinkles of any kind, gummy bears, real flowers.