What you have to do is enter a property with a molehill, walk up to it, and hit the prompt to destroy it. Everybody's favourite friend, Mr. Resetti, will pop out and say hello, and give you the achievement as a parting present.
Edit: While I originally thought this was random, NONEsoJIVE has added that you can find one on “Camping Bungalow”. When you buy the property and load in you will be alerted to a molehill infestation.
Edit 2: As seen in the comments, there have been issues with “Camping Bungalow”. It does appear to be random, as I originally thought.