Put the Oreos into a zip-lock bag/a plastic bag and bash with a rolling pin to turn it into crumbs - you can also use a food processor if you have one and don't mind the extra cleaning! (You can save half a Tsp of the crumbs for decorating at the end if you wish!)
Melt the butter into a bowl for about 30-40 seconds in the microwave until fully melted, and then combine with the Oreos and mix
Pour the Oreo crumbs mixture into the bottom of a small glass to create your base, gently pushing down with the back of the spoon to flatten it (but not too hard, or else it'll be really hard to break into when its out of the fridge)