I use silver powder that I get from work as a byproduct of developing film negatives. It's supposedly fine silver (according to my salesman) and it works well. A lot of people use fine silver bezel wire. A little goes a long ways.
Heat the piece to be fumed with a soft, fluffly flame. Heat the tip of a borosilicate rod and pick up the silver on the end. Change to a reducing flame, hold the silver in the flame about 1" from the tip of the torch. Let it start to melt. You should see the fumes start to develop. move the piece to be fumed into the flame towards the tip of the flame. The silver will start to deposit on the surface of the glass. You need to be careful not to burn your silver off. If you do, changing back to a reducing flame will usually bring it back.