For this achievement you have to fly as high as possible and then freefall the longest possible time to open your parachute in the last moment and land safe.
This achievement will become obtainable when you have access to the parachute, after the mission "High Dive".
Make sure you have your parachute on and enter any helicopter, you will find various within Liberty City.
This is a handy map for locations in Liberty City:http://grandtheftauto.ign.com/maps/1/Liberty-City-Map
Then fly as high as possible and watch your altimeter on the radar, also be sure to fly over water, just to be on the safe side.
I used Yusuf's Buzzard, this one will be unlocked after you finished the storyline. There is also a parachute nearby.
The most difficult thing is the correct timing.
Since I just got this achievement, I used a stopwatch to get the exact time.
The moment you press "Y" to jump out of the helicopter, start your stopwatch. Then only look at that stopwatch and press "A" after 14,9 seconds. Then you should get the achievement when landing in the water.
I did it nearby where the buzzard spawns but it should work at any place if you fly over water.
I hope I could help you with this achievement :D.
Feel free to give your comment if it works or not.