Freefall more than 100m and live (Single Player only).

Throughout the game you'll come across gliders which are in most cases on the high hills of the island. Only a few of these though are high enough to reach the 100m mark. The earliest and best one is on the upper left side of the island which you'll head over to early on in the game. Head to these coordinates - X: 442.4, Y: 651.1 - to find the glider. Now grab hold of it with X and glide out to the water in front of you.

Don't move the gliders position and just head straight. Keep looking down with the until you're over the water and when you're in a good position, hold X to drop from the sky. Once you hit the water, the achievement will unlock.