You locate a vole tunnel and dig out enough of the roof to put the trap in it perpendicular to the walls. Then you cover the hole with a board or something. Only the voles will be caught.

Can you make your own slug cream by gathering up a bunch of them and running them through the blender? Or is there some secret to preparation that I'm missing?

As I noted, we don't have many slugs or snails here because it's dry -- we average only 17 inches of precipitation a year.