Position the saw horses so that the area to be cut will be hanging off one end. The aluminum plate should rest securely on the saw horses and there should not be too much extra hanging off the end. Put on gloves so that you do not accidentally cut yourself on the aluminum's edges if they are sharp.
Ensure the diamond plate is on its backside. This will be the side you'll be cutting on. The front side will have the little diamond "bumps" on it, but the back side will be smooth and a perfect medium to cut upon.
Measure out your pattern or piece that needs to be cut. Visualize or mark on the aluminum with a lead pencil to indicate where your first cut will occur. Allow this part of the plate to extend over the side of the saw horse. Use the ruler to make sure your lines are straight and measured accurately.
Cut with your jigsaw on the drawn line. Wear your safety glasses from here on out. A 48 tooth blade will cut smoothly, but it will also cut slowly. Follow the line you have made, supplying only enough pressure on the jigsaw to keep the blade moving.
Reposition and cut again, if you need to shape the diamond aluminum plate further.