When working with reinforcements in the form of a woven mat, cutting is necessary to allow for proper orientation, workability, and strength. The most common way to cut these mats is with industrial scissors. Other methods include rotary cutters, die cutters, and electric shears, but a good pair of oversized, resharpenable, thru-hardened shears (scissors) are the best way to get started.
Woven mats can be cut to size in the dry stage -before the resin is applied- or in the wet stage, when resin is flowing freely.
There are benefits and drawbacks to both, and operators usually find their own preferred technique. Cutting mats in the dry stage requires that it gets put together correctly when it is in the wet stage. Handling dry fiberglass is typically more itchy than wet fiberglass, which is sticky. Scissors used to cut wet reinforcement must be properly cleaned in order to be used again.
Many claims are made about the difficulty of cutting kevlar reinforcement. This can be remedied by using a dedicated pair of sharpened scissors only for kevlar. Kevlar is a material that requires a different angle on the blade in order to cut it. I have demonstrated to fellow workers how a fresh pair of scissors will cut kevlar all day, then cut a bunch of fiberglass. But when going back to the kevlar, the scissors will not cut it. Carbon Fiber falls into the category side of fiberglass where it will dull the blades and not go back.