The simplest way is probably to put the Nilla wafers in a Ziploc bag and then use a rolling pin to smash them. The flat end of a meat mallet would also work. Alternatively, you could pulse them in a blender or food processor. There are a couple of potential issues with that though. The cookies can get stuck under the blade and need to be dislodged with a utensil. Also, it's easy to over-blend and then you'll end up with a fine sand texture instead of the slightly coarser crumb which offers a good crunch.