Must be done on the Narco Road DLC. They are accessed separately in the main menu.
Video guide by @Maka91 who uses the same method I did and explains it better :)
A little tricky but not too bad if you go to a certain spot in Media Luna, the starting area of this DLC.
As you fly around in a helicopter on the DLC you might notice some "aerobatic" challenges which require you to fly through a single blue circle. EDIT: This achievement requires you to do one of these whilst free-falling - i.e. out of the helicopter and free falling without your parachute. Despite the description its also been confirmed that a glided parachute will work too (thanks to UnsaferLlama7 & Tulukaruq907 for this find.
The perfect spot is by fast travelling to "Eddie's Campsite" if you want to get it done asap. If you zoom in on your tacmap you'll see one of the aforementioned challenges. Grab a chopper and head to it.
When you freefall, you automatically rotate 180 degrees. The best tactic is to fly above and past the blue circle a couple of hundred metres, then freefall. It will rotate you 180 degrees and the blue circle should be dead infront of you. Now try and frefall into the blue circle. ou'll get a feel for the timing, but try and get a fair bit of height because you will fall fast Will pop as soon as you've done so, even if you die afterwards.