First, you will need a set (or one or two) metallic markers. It is best to start out with Brand New ones as the tips won't already be pressed on from writing.
Next, take a pair of Scissor (yes, really!) and using the Edge of one of the blades, press DOWN into the tip of the marker until you get a nice V.
Now, to use them, you just place your card in the V and slide the marker along the edge of the card....Voilá....you have the perfect edge on your card!