I agree with Cheryl about the hot knife (mine's called a "stencil cutter") as far as ease of cutting, but strong shears or a sharp xacto knife should work, too. If you use the xacto, I would use several passes to get deeper each time, rather than trying to go through the whole thing at once. If you sand the cut edges to "round" them, they shouldn't leave any lines. Let us know how it works out!
Happy for no reason...