Summarize the following:
If you're cutting the rigid foam for a specific project, measure the length and width you will need it to be. Compare these dimensions to the length and width that your rigid foam insulation currently has to make sure it's long and wide enough. For more precise measurements, check the length and width 2-3 times. Place your straightedge ruler on the rigid foam insulation and use it as a guide while you make the mark. Draw a line with your permanent marker along the side of your straightedge, then check the marking to make sure it's precise. It's better for your markings to be a few inches or centimeters too large at first than too small. You can always cut more foam later, if necessary. Keeping the board flat while you cut will help you make a precise cut. If you don't have a work table, find a flat and smooth surface for cutting your foam insulation. Do not cut rigid foam insulation on decorative tables unless you lay out a protective barrier, like a knife block. You risk accidentally scratching the table. Use rubber clamps to secure both sides of the insulation to the work surface. Secure the straightedge alongside the cutting line to help you make a precise incision. This will keep the rigid foam insulation from shifting or sliding while you cut it.

Summary:
Measure your cut with a straightedge ruler. Mark your cutting line on the foam with a permanent marker. Lay the rigid foam insulation on a flat surface. Clamp the rigid foam to the flat surface.