You only take a small piece from drywall in order to have enough material to send to a lab. Using a chisel or a utility knife, you'll lightly trace the tip around an outline with the right dimensions to loosen the sample. Then, you'll put a wet paper towel between the mouth of some pliers, which will keep any possible asbestos dust from falling down. When you carefully pull the small piece off with the pliers, you'll prevent any exposure to asbestos. You should then cover the spot where you took the sample with latex paint.