go to www.quiltingstencils.com (The Stencil Co) there wil be a picture in their catalog of the double bladed stencil cutting knife. You can use a single blade knife like Xacto but you have to cut two lines. The double blade is much easier to use.
They are not generally available in hobby shops as the company Olfa has discontinued the manfacturer. Another hint. You will need a flexible plastic Neither the Xacto single blade or the Olfa double blade will cut the stiffer plastic. The DBK from The Stencil co or similar flexible plastic will be easier to cut. If you should contact The Stencil Co, ask them for the instruction sheet for using the double blade knifes.