You can make templates out of cardboard or Luan plywood and pre fit them first. Once they are correct, use them to mark the cut lines on your purchased panels.

"Measure twice, cut once" is the rule. But if you didn't follow it, "I cut it three times and it's still too short" often becomes the result.

When you cut your panels using the templates, cut them slightly oversize. You can always then trim them down to an accurate fit during installation.

Gard

A fine tooth "plywood" blade on a circular saw works well. Make sure you cut your lines straight, as the blade has little offset in the teeth and can cause kick back if it binds.

A fine tooth metal cutting blade installed on a jig saw will allow you to cut tight curves. A dry wall bit installed in a high speed cut out tool will easily cut any profile in fiberglass, as will a straight 1/4" bit installed in a router run at its highest rpm.

The later tools will have to be used with a guide to produce straight cuts. This can be as simple as a piece of wood held in place with clamps.

Gard

Try putting some masking tape down first then draw your cutting line on that. I used that on mine.  It help keep it from fraying.