Cutting pieces from an existing hairpiece is easier than it looks.

I use a sharp triangular hobby knife and cut from the bottom. As long as you insert only the tip of the knife and make sure there is no tension on any of the hairs in the area where you are cutting you won't cut more than a few hairs. It takes a little practice to develop a technique for holding the hairpiece in one hand without pulling on any of the hairs that are in the danger zone while you use your other hand to cut, but after you have done it once it's like riding a bicycle from that point on.