Fiberglass resin is a glass-reinforced plastic or fiber-reinforced polymer. Cutting it is a fairly easy task, but care should be taken not just so that you do not damage the fiberglass but also for your own safety.
Step 1 - Protect Yourself
Before you begin cutting the fiberglass you should protect yourself as much as you can. While you are cutting into the fiberglass tiny, but sometimes sharp, fragments of fiberglass can flick off and end up in your skin, eyes or mouth. Wear long pants, long-sleeve shirt, gloves, mask and goggles before you even begin your cut to make sure you are protected.
Step 2 - Clamp the Fiberglass
Use clamps to firmly secure the panel to something steady like a sawhorse. If you want use a ruler and a fine-tipped marker to draw a line where you will be cutting to ensure a straight line.
Step 3 - Make Your Cut
MAKE SURE that the saw blade that you are using on your circular saw is fine-bladed. If the blade is not sharp and narrow then the cut you will make will most likely be jagged and rough. If you end up with such a cut then there is not much else to do but make another cut with a finer blade.