You can buy them already chopped at the store, but if you don't want to spend the extra cost and are willing to do it by hand, the easiest way to chop them is by adding them to the food processor and pulsing for a short time. Don't do it for too long, or it will break them into pieces that are too small. You can also add them to a plastic bag and roll a rolling pin over the top to break them into pieces.