Make and bake the cake according to the directions on the box.
After the cake has cooled, use a wooden spoon handle (or other similar size utensil), and poke holes all over the cake down to the bottom.
Then mix the pudding powder with the milk and pour over the cake, using a spoon to work it down into the holes.
Chill in the fridge for at least a couple of hours.
Then spread the whipped topping over the pudding layer and chill again if desired.
Crush your vanilla wafers in a ziploc bag.
Now some spread the crumbles all over the cake, but since I knew we wouldn't be eating the cake all at once, I didn't want the cookie crumbles to get soggy, so I only put them on as I served individual pieces, and I did the same with the banana slices.