you don't use a craft knife with a light box unless it is a glass one and you actually want to cut the image.
most use the light box and brass stencils for dry embossing, and you will need a stylus instead of the knife.
here are some videos, i tried to find a tutorial in the resource section but there isn't one that i can see.
http://www.youtube.com/results?searc...rch_type=&aq=0