Draw your selected design on white paper. You can print off free designs online or draw your own. Draw the lines thick to make the stencil easier to cut.
Cut out the white paper designs with scissors (or Stanley knife if the design is precise and complex). Keep the cut line inside the lines you have drawn, not around.
Place your white paper stencil onto cardboard, and trace the stencil design with a marker pen. Thin cardboard, such as cereal box cardboard, works especially well.
Place your cutting mat (or thick cardboard) on a flat surface. Using your Stanley knife, carefully cut out your cardboard stencil. Again you must make sure you cut into the markings and not around.
Check your stencil once it is cut to ensure there are not sharp edges that would effect the final results. When you are happy with your stencil design place masking tape around the far edges to stick the stencil onto your car.