How difficult is it to cut a fabric strong enough to stop bullets? It can be pretty tough. But like many things in life, having the proper tools available can make the job much simpler. For cutting Kevlar, there are purpose-built carbide or ceramic shears available either online or at high-end hardware stores that cater to the needs of engineering and aerospace industries. If the scale of the job is small enough, a nice pair of sewing scissors or a heavy-duty utility knife will usually work in a pinch -- but don't expect the blades to maintain their sharpness for long.
Determine the scale of the job. If simply trying to cut out a few small patches of lightweight Kevlar for one-time-use application, it would be most timely and economical to buy a pair of scissors or a few blades for a utility knife with a view to disposing of them afterward. For a bigger job or for continued use, a high quality pair of Kevlar shears can be purchased for between £26 and £52, as of 2011, depending on the design and quality of the shear. An ordinary pair of scissors can also be converted into Kevlar shears by roughing up the blades on a disc sander. To accomplish this, take turns holding each blade of the scissors perpendicular to the disc sander and then draw it across the disc.
Measure the size of Kevlar piece to be cut.
Cut the Kevlar like any other fabric, using scissors or Kevlar shears. Good shears will leave a very clean paper-like cut that won't need any touch-up work. If there is any Kevlar fuzz along the edges of the cut, sand off with 220-grit sandpaper or remove with a utility knife.
Do not use Kevlar shears to cut fibreglass. The edge of the shears will be worn down very quickly, and fibreglass already cuts easily enough with even a poor pair of scissors.