1. In the DLC you need to first unlock the "Summer Camp"/"Adventure Area" for 300 BC (i assume Bid coins?)
To get those you need to either sell stuff, win auctions or craft Items.
2. Go on top of the "Mountain" in the Middle of the Summer Camp, you will find Stairs when you enter the Area and go the path to the right around the Hill.
3. You will find 6 Blue "Porta-Pottys"
One of the doors should be slightly open, when you left click, it will close and another one opens.
Do this through all 6 Toilets and you will get the achievement and hear a farttastic melody
In case the door isnt open:
I pressed the lever on the left of the 6 toilets before checking if the toilets did anything.
Its on default set to "classical" (lever up)
And thats when you can get the achievement, if its on Freestyle you are free to make a fart melody of your own.