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!
I'm blogging again: random.on purpose.
Within you there is a solitude and peace to which you can retreat at any time and be yourself...