The easiest and most efficient method is to use a food processor. Pulse to break the cookies down and then run it until the cookies are finely ground. If you don't have a food processor you can place the cookies in a ziplock bag and pound them with a rolling pin or mallet. This will take longer and it's important that you ensure the cookies are finely ground so they stick together in the crust. The last option is to buy chocolate cookie crumbs.